/* ============================================
   DESIGN SYSTEM - Learning-Focused UI
   Global styles shared across all chapters
   ============================================ */

:root {
    /* Core Colors */
    --rust-orange: #f74c00;
    --rust-orange-light: #ff6b2b;
    --rust-dark: #0d1117;
    --rust-darker: #010409;
    --rust-light: #f0f6fc;

    /* Semantic Colors */
    --success: #3fb950;
    --success-bg: rgba(63, 185, 80, 0.1);
    --error: #f85149;
    --error-bg: rgba(248, 81, 73, 0.1);
    --warning: #d29922;
    --warning-bg: rgba(210, 153, 34, 0.1);
    --info: #58a6ff;
    --info-bg: rgba(88, 166, 255, 0.1);

    /* Code Colors (One Dark inspired) */
    --code-bg: #161b22;
    --code-surface: #21262d;
    --keyword: #ff7b72;
    --string: #a5d6ff;
    --function: #d2a8ff;
    --number: #79c0ff;
    --comment: #8b949e;
    --type: #ffa657;
    --macro: #7ee787;
    --variable: #ffa198;
    --attribute: #f472b6;

    /* Layout */
    --max-width: 900px;
    --spacing-xs: 4px;
    --spacing-sm: 8px;
    --spacing-md: 16px;
    --spacing-lg: 24px;
    --spacing-xl: 40px;
    --spacing-2xl: 64px;

    /* Typography */
    --font-sans: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
    --font-mono: 'JetBrains Mono', 'Fira Code', 'Consolas', monospace;

    /* Animation */
    --transition-fast: 150ms ease;
    --transition-normal: 250ms ease;
    --transition-slow: 400ms ease;

    /* Chapter colors (overridden per chapter) */
    --chapter-primary: var(--rust-orange);
    --chapter-primary-dark: #c73e00;
    --chapter-primary-light: var(--rust-orange-light);
    --chapter-gradient: linear-gradient(135deg, var(--rust-orange) 0%, #c73e00 100%);
}

/* Chapter-specific colors */
.chapter-2 {
    --chapter-primary: #f74c00;
    --chapter-primary-dark: #c73e00;
    --chapter-primary-light: #ff6b2b;
    --chapter-gradient: linear-gradient(135deg, #f74c00 0%, #c73e00 100%);
}

.chapter-3 {
    --chapter-primary: #7c3aed;
    --chapter-primary-dark: #6d28d9;
    --chapter-primary-light: #a78bfa;
    --chapter-gradient: linear-gradient(135deg, #7c3aed 0%, #6d28d9 100%);
}

.chapter-4 {
    --chapter-primary: #10b981;
    --chapter-primary-dark: #059669;
    --chapter-primary-light: #34d399;
    --chapter-gradient: linear-gradient(135deg, #10b981 0%, #059669 100%);
}

.chapter-5 {
    --chapter-primary: #06b6d4;
    --chapter-primary-dark: #0891b2;
    --chapter-primary-light: #22d3ee;
    --chapter-gradient: linear-gradient(135deg, #06b6d4 0%, #0891b2 100%);
}

.chapter-6 {
    --chapter-primary: #f43f5e;
    --chapter-primary-dark: #e11d48;
    --chapter-primary-light: #fb7185;
    --chapter-gradient: linear-gradient(135deg, #f43f5e 0%, #e11d48 100%);
}

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

body {
    font-family: var(--font-sans);
    background: var(--rust-dark);
    color: var(--rust-light);
    line-height: 1.7;
    font-size: 16px;
    -webkit-font-smoothing: antialiased;
}

/* ============================================
   LAYOUT COMPONENTS
   ============================================ */

.container {
    max-width: var(--max-width);
    margin: 0 auto;
    padding: 0 var(--spacing-lg);
}

/* Hero Section */
.hero {
    background: var(--chapter-gradient);
    padding: var(--spacing-2xl) var(--spacing-lg);
    position: relative;
    overflow: hidden;
}

.hero::before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background: url("data:image/svg+xml,%3Csvg width='60' height='60' viewBox='0 0 60 60' xmlns='http://www.w3.org/2000/svg'%3E%3Cg fill='none' fill-rule='evenodd'%3E%3Cg fill='%23ffffff' fill-opacity='0.05'%3E%3Cpath d='M36 34v-4h-2v4h-4v2h4v4h2v-4h4v-2h-4zm0-30V0h-2v4h-4v2h4v4h2V6h4V4h-4zM6 34v-4H4v4H0v2h4v4h2v-4h4v-2H6zM6 4V0H4v4H0v2h4v4h2V6h4V4H6z'/%3E%3C/g%3E%3C/g%3E%3C/svg%3E");
    opacity: 0.5;
}

.hero-content {
    max-width: var(--max-width);
    margin: 0 auto;
    position: relative;
    z-index: 1;
}

.hero-badge {
    display: inline-flex;
    align-items: center;
    gap: var(--spacing-sm);
    background: rgba(255,255,255,0.2);
    padding: var(--spacing-xs) var(--spacing-md);
    border-radius: 20px;
    font-size: 0.85em;
    font-weight: 500;
    margin-bottom: var(--spacing-md);
    backdrop-filter: blur(10px);
}

.hero h1 {
    font-size: clamp(2rem, 5vw, 3rem);
    font-weight: 700;
    margin-bottom: var(--spacing-sm);
    letter-spacing: -0.02em;
}

.hero-subtitle {
    font-size: 1.2em;
    opacity: 0.9;
    margin-bottom: var(--spacing-xl);
    max-width: 600px;
}

.hero-nav {
    margin-bottom: var(--spacing-xl);
}

.hero-nav a {
    display: inline-flex;
    align-items: center;
    gap: var(--spacing-sm);
    color: rgba(255,255,255,0.8);
    text-decoration: none;
    font-size: 0.9em;
    transition: color var(--transition-fast);
}

.hero-nav a:hover { color: white; }

/* Learning Objectives */
.objectives {
    background: rgba(255,255,255,0.1);
    border-radius: 12px;
    padding: var(--spacing-lg);
    backdrop-filter: blur(10px);
}

.objectives h3 {
    font-size: 0.9em;
    text-transform: uppercase;
    letter-spacing: 0.1em;
    opacity: 0.8;
    margin-bottom: var(--spacing-md);
    display: flex;
    align-items: center;
    gap: var(--spacing-sm);
}

.objectives-list {
    display: grid;
    gap: var(--spacing-sm);
    list-style: none;
}

.objectives-list li {
    display: flex;
    align-items: flex-start;
    gap: var(--spacing-sm);
    font-size: 0.95em;
}

.objectives-list .check {
    color: rgba(255,255,255,0.6);
    flex-shrink: 0;
    margin-top: 2px;
}

.objectives-list li.completed .check {
    color: #7ee787;
}

/* ============================================
   PROGRESS & NAVIGATION
   ============================================ */

.lesson-nav {
    position: sticky;
    top: 0;
    z-index: 100;
    background: rgba(13, 17, 23, 0.95);
    backdrop-filter: blur(10px);
    border-bottom: 1px solid rgba(255,255,255,0.1);
    padding: var(--spacing-md) 0;
}

.nav-inner {
    max-width: var(--max-width);
    margin: 0 auto;
    padding: 0 var(--spacing-lg);
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: var(--spacing-md);
}

.progress-wrapper {
    flex: 1;
    max-width: 400px;
}

.progress-label {
    display: flex;
    justify-content: space-between;
    font-size: 0.8em;
    color: var(--comment);
    margin-bottom: var(--spacing-xs);
}

.progress-bar {
    height: 6px;
    background: var(--code-surface);
    border-radius: 3px;
    overflow: hidden;
}

.progress-fill {
    height: 100%;
    background: linear-gradient(90deg, var(--chapter-primary), var(--chapter-primary-light));
    border-radius: 3px;
    transition: width var(--transition-normal);
}

.step-dots {
    display: flex;
    gap: var(--spacing-sm);
}

.step-dot {
    width: 10px;
    height: 10px;
    border-radius: 50%;
    background: var(--code-surface);
    border: 2px solid transparent;
    cursor: pointer;
    transition: all var(--transition-fast);
}

.step-dot:hover {
    background: var(--comment);
}

.step-dot.active {
    background: var(--chapter-primary);
    border-color: var(--chapter-primary-light);
    transform: scale(1.2);
}

.step-dot.completed {
    background: var(--success);
}

/* ============================================
   LESSON SECTIONS
   ============================================ */

.lesson-content {
    padding: var(--spacing-xl) 0 var(--spacing-2xl);
}

.lesson-section {
    display: none;
    animation: fadeSlideIn 0.4s ease;
}

.lesson-section.active {
    display: block;
}

@keyframes fadeSlideIn {
    from {
        opacity: 0;
        transform: translateY(20px);
    }
    to {
        opacity: 1;
        transform: translateY(0);
    }
}

/* Section Header */
.section-header {
    margin-bottom: var(--spacing-xl);
}

.section-step {
    display: inline-flex;
    align-items: center;
    justify-content: center;
    width: 32px;
    height: 32px;
    background: var(--chapter-primary);
    border-radius: 50%;
    font-weight: 700;
    font-size: 0.9em;
    margin-bottom: var(--spacing-md);
}

.section-header h2 {
    font-size: 1.8em;
    font-weight: 700;
    color: var(--rust-light);
    margin-bottom: var(--spacing-sm);
}

.section-header .lead {
    font-size: 1.1em;
    color: var(--comment);
    max-width: 600px;
}

/* Content Blocks */
.content-block {
    margin-bottom: var(--spacing-xl);
}

.content-block h3 {
    font-size: 1.3em;
    color: var(--rust-light);
    margin-bottom: var(--spacing-md);
    display: flex;
    align-items: center;
    gap: var(--spacing-sm);
}

.content-block p {
    color: rgba(240, 246, 252, 0.85);
    margin-bottom: var(--spacing-md);
}

/* ============================================
   INTERACTIVE CODE EXPLORER
   ============================================ */

.code-explorer {
    background: var(--code-bg);
    border-radius: 12px;
    overflow: hidden;
    margin: var(--spacing-lg) 0;
    border: 1px solid var(--code-surface);
}

.code-explorer-header {
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding: var(--spacing-md) var(--spacing-lg);
    background: var(--code-surface);
    border-bottom: 1px solid rgba(255,255,255,0.05);
}

.code-filename {
    display: flex;
    align-items: center;
    gap: var(--spacing-sm);
    font-family: var(--font-mono);
    font-size: 0.85em;
    color: var(--comment);
}

.code-filename .icon {
    color: var(--chapter-primary);
}

.code-actions {
    display: flex;
    gap: var(--spacing-sm);
}

.code-action-btn {
    background: rgba(255,255,255,0.05);
    border: 1px solid rgba(255,255,255,0.1);
    border-radius: 6px;
    padding: var(--spacing-xs) var(--spacing-md);
    color: var(--comment);
    font-size: 0.8em;
    cursor: pointer;
    transition: all var(--transition-fast);
}

.code-action-btn:hover {
    background: rgba(255,255,255,0.1);
    color: var(--rust-light);
}

.code-body {
    display: flex;
}

.line-numbers {
    padding: var(--spacing-lg);
    background: rgba(0,0,0,0.2);
    text-align: right;
    user-select: none;
    border-right: 1px solid rgba(255,255,255,0.05);
}

.line-numbers span {
    display: block;
    font-family: var(--font-mono);
    font-size: 0.85em;
    line-height: 1.8;
    color: var(--comment);
    opacity: 0.5;
}

.code-content {
    flex: 1;
    padding: var(--spacing-lg);
    overflow-x: auto;
}

.code-line {
    font-family: var(--font-mono);
    font-size: 0.9em;
    line-height: 1.8;
    padding: 2px var(--spacing-sm);
    margin: 0 calc(-1 * var(--spacing-sm));
    border-radius: 4px;
    cursor: pointer;
    transition: all var(--transition-fast);
    position: relative;
    white-space: pre;
}

.code-line:hover {
    background: rgba(var(--chapter-primary), 0.1);
}

.code-line.highlighted {
    background: rgba(var(--chapter-primary), 0.15);
    border-left: 3px solid var(--chapter-primary);
    padding-left: calc(var(--spacing-sm) - 3px);
}

.code-line.active {
    background: rgba(88, 166, 255, 0.1);
    border-left: 3px solid var(--info);
    padding-left: calc(var(--spacing-sm) - 3px);
}

.code-line.error-line {
    background: var(--error-bg);
    border-left: 3px solid var(--error);
    padding-left: calc(var(--spacing-sm) - 3px);
}

/* Line Annotation */
.line-annotation {
    display: none;
    background: var(--code-surface);
    border: 1px solid rgba(255,255,255,0.1);
    border-radius: 8px;
    padding: var(--spacing-md);
    margin: var(--spacing-sm) 0;
    font-family: var(--font-sans);
    font-size: 0.9em;
    color: var(--rust-light);
    animation: slideDown 0.2s ease;
}

.line-annotation.visible {
    display: block;
}

@keyframes slideDown {
    from { opacity: 0; transform: translateY(-10px); }
    to { opacity: 1; transform: translateY(0); }
}

.line-annotation h4 {
    color: var(--info);
    font-size: 0.95em;
    margin-bottom: var(--spacing-sm);
}

.line-annotation code {
    background: rgba(255,255,255,0.1);
    padding: 2px 6px;
    border-radius: 4px;
    font-family: var(--font-mono);
    font-size: 0.9em;
}

/* Syntax Highlighting */
.kw { color: var(--keyword); }
.str { color: var(--string); }
.fn { color: var(--function); }
.num { color: var(--number); }
.cmt { color: var(--comment); font-style: italic; }
.ty { color: var(--type); }
.mac { color: var(--macro); }
.var { color: var(--variable); }
.attr { color: var(--attribute); }
.variant { color: #c4b5fd; }
.field { color: #7dd3fc; }

/* ============================================
   PREDICT & VERIFY CHALLENGES
   ============================================ */

.challenge {
    background: var(--info-bg);
    border: 1px solid rgba(88, 166, 255, 0.3);
    border-radius: 12px;
    padding: var(--spacing-lg);
    margin: var(--spacing-lg) 0;
}

.challenge-header {
    display: flex;
    align-items: center;
    gap: var(--spacing-sm);
    margin-bottom: var(--spacing-md);
}

.challenge-icon {
    width: 28px;
    height: 28px;
    background: var(--info);
    border-radius: 50%;
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 0.9em;
}

.challenge-title {
    font-weight: 600;
    color: var(--info);
}

.challenge-question {
    font-size: 1.1em;
    margin-bottom: var(--spacing-lg);
}

.challenge-options {
    display: grid;
    gap: var(--spacing-sm);
    margin-bottom: var(--spacing-md);
}

.challenge-option {
    display: flex;
    align-items: center;
    gap: var(--spacing-md);
    padding: var(--spacing-md);
    background: rgba(255,255,255,0.05);
    border: 2px solid transparent;
    border-radius: 8px;
    cursor: pointer;
    transition: all var(--transition-fast);
    font-family: var(--font-mono);
}

.challenge-option:hover {
    background: rgba(255,255,255,0.08);
    border-color: rgba(255,255,255,0.1);
}

.challenge-option.selected {
    border-color: var(--info);
    background: rgba(88, 166, 255, 0.1);
}

.challenge-option.correct {
    border-color: var(--success);
    background: var(--success-bg);
}

.challenge-option.incorrect {
    border-color: var(--error);
    background: var(--error-bg);
}

.option-marker {
    width: 24px;
    height: 24px;
    border: 2px solid var(--comment);
    border-radius: 50%;
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 0.8em;
    flex-shrink: 0;
}

.challenge-option.selected .option-marker {
    border-color: var(--info);
    background: var(--info);
    color: white;
}

.challenge-option.correct .option-marker {
    border-color: var(--success);
    background: var(--success);
}

.challenge-option.incorrect .option-marker {
    border-color: var(--error);
    background: var(--error);
}

.challenge-feedback {
    display: none;
    padding: var(--spacing-md);
    border-radius: 8px;
    margin-top: var(--spacing-md);
}

.challenge-feedback.visible {
    display: block;
    animation: fadeSlideIn 0.3s ease;
}

.challenge-feedback.success {
    background: var(--success-bg);
    border: 1px solid rgba(63, 185, 80, 0.3);
}

.challenge-feedback.error {
    background: var(--error-bg);
    border: 1px solid rgba(248, 81, 73, 0.3);
}

/* ============================================
   MEMORY VISUALIZATION
   ============================================ */

.memory-visual {
    background: var(--code-bg);
    border-radius: 12px;
    padding: var(--spacing-lg);
    margin: var(--spacing-lg) 0;
    border: 1px solid var(--code-surface);
}

.memory-visual-header {
    display: flex;
    align-items: center;
    justify-content: space-between;
    margin-bottom: var(--spacing-lg);
}

.memory-visual-title {
    display: flex;
    align-items: center;
    gap: var(--spacing-sm);
    font-weight: 600;
    color: var(--comment);
}

.memory-stack {
    display: flex;
    flex-wrap: wrap;
    gap: var(--spacing-lg);
    justify-content: center;
}

.memory-slot {
    background: var(--code-surface);
    border: 2px solid var(--comment);
    border-radius: 8px;
    padding: var(--spacing-md);
    min-width: 140px;
    text-align: center;
    transition: all var(--transition-normal);
}

.memory-slot.immutable {
    border-color: var(--info);
}

.memory-slot.mutable {
    border-color: var(--rust-orange);
    box-shadow: 0 0 20px rgba(247, 76, 0, 0.2);
}

.memory-slot.const {
    border-color: #7c3aed;
    box-shadow: 0 0 20px rgba(124, 58, 237, 0.2);
}

.memory-slot.shadowed {
    opacity: 0.4;
    transform: scale(0.9);
}

.memory-slot.updating {
    animation: pulse 0.5s ease;
}

.memory-slot.moved {
    border-color: var(--error);
    opacity: 0.5;
}

.memory-slot.borrowed {
    border-color: var(--success);
    box-shadow: 0 0 20px rgba(63, 185, 80, 0.2);
}

@keyframes pulse {
    0%, 100% { transform: scale(1); }
    50% { transform: scale(1.05); }
}

@keyframes shake {
    0%, 100% { transform: translateX(0); }
    25% { transform: translateX(-5px); }
    75% { transform: translateX(5px); }
}

.slot-name {
    font-family: var(--font-mono);
    font-size: 0.9em;
    color: var(--function);
    margin-bottom: var(--spacing-sm);
}

.slot-value {
    font-family: var(--font-mono);
    font-size: 1.4em;
    color: var(--string);
    padding: var(--spacing-sm);
    background: rgba(0,0,0,0.3);
    border-radius: 4px;
    min-height: 40px;
    display: flex;
    align-items: center;
    justify-content: center;
}

.slot-type {
    font-family: var(--font-mono);
    font-size: 0.75em;
    color: var(--type);
    margin-top: var(--spacing-sm);
}

.slot-badge {
    font-size: 0.7em;
    padding: 2px 8px;
    border-radius: 10px;
    margin-top: var(--spacing-sm);
}

.slot-badge.mut {
    background: rgba(247, 76, 0, 0.2);
    color: var(--rust-orange);
}

.slot-badge.imm {
    background: rgba(88, 166, 255, 0.2);
    color: var(--info);
}

.slot-badge.const-badge {
    background: rgba(124, 58, 237, 0.2);
    color: #a78bfa;
}

/* String Memory Layout */
.string-memory {
    display: flex;
    align-items: center;
    gap: var(--spacing-lg);
    flex-wrap: wrap;
    justify-content: center;
}

.memory-table {
    background: var(--code-surface);
    border-radius: 8px;
    border: 2px solid var(--chapter-primary);
    overflow: hidden;
    min-width: 120px;
}

.memory-table-header {
    background: rgba(16, 185, 129, 0.2);
    padding: var(--spacing-sm) var(--spacing-md);
    font-weight: 600;
    text-align: center;
    color: var(--chapter-primary);
    font-size: 0.9em;
}

.memory-table-row {
    display: flex;
    justify-content: space-between;
    padding: var(--spacing-sm) var(--spacing-md);
    border-top: 1px solid rgba(255,255,255,0.1);
}

.memory-table-row .label {
    color: var(--comment);
    font-size: 0.85em;
}

.memory-table-row .value {
    color: var(--string);
    font-family: var(--font-mono);
    font-size: 0.85em;
}

.memory-table.heap-data {
    border-color: var(--warning);
}

.memory-table.heap-data .memory-table-header {
    background: rgba(210, 153, 34, 0.2);
    color: var(--warning);
}

.memory-table.moved {
    border-color: var(--error);
    opacity: 0.5;
}

.memory-table.moved .memory-table-header {
    background: rgba(248, 81, 73, 0.2);
    color: var(--error);
}

.memory-table.stack-data {
    border-color: var(--chapter-primary);
}

.memory-arrow {
    font-size: 1.5em;
    color: var(--chapter-primary);
}

.heap-bytes-container {
    padding: var(--spacing-md);
}

.heap-bytes {
    display: flex;
    gap: 4px;
}

.heap-byte {
    background: rgba(255,255,255,0.1);
    padding: 8px 12px;
    border-radius: 4px;
    text-align: center;
}

.heap-byte .char {
    color: var(--string);
    font-family: var(--font-mono);
    display: block;
}

.heap-byte .index {
    color: var(--comment);
    font-size: 0.7em;
}

.heap-value {
    padding: var(--spacing-sm);
    font-family: var(--font-mono);
    text-align: center;
}

/* Rules Box */
.rules-box {
    background: rgba(16, 185, 129, 0.1);
    border: 2px solid var(--chapter-primary);
    border-radius: 12px;
    padding: var(--spacing-lg);
    margin: var(--spacing-lg) 0;
}

.rules-box h4 {
    color: var(--chapter-primary);
    margin-bottom: var(--spacing-md);
}

.rules-list {
    list-style: none;
    counter-reset: rule;
}

.rules-list li {
    counter-increment: rule;
    margin-bottom: var(--spacing-md);
    display: flex;
    align-items: flex-start;
    gap: var(--spacing-md);
}

.rules-list li::before {
    content: counter(rule);
    background: var(--chapter-primary);
    color: white;
    width: 28px;
    height: 28px;
    border-radius: 50%;
    display: flex;
    align-items: center;
    justify-content: center;
    font-weight: 700;
    flex-shrink: 0;
}

/* ============================================
   INFO BOXES
   ============================================ */

.info-box {
    border-radius: 8px;
    padding: var(--spacing-md) var(--spacing-lg);
    margin: var(--spacing-lg) 0;
    display: flex;
    gap: var(--spacing-md);
}

.info-box.tip {
    background: var(--success-bg);
    border-left: 3px solid var(--success);
}

.info-box.warning {
    background: var(--warning-bg);
    border-left: 3px solid var(--warning);
}

.info-box.note {
    background: var(--info-bg);
    border-left: 3px solid var(--info);
}

.info-box.error {
    background: var(--error-bg);
    border-left: 3px solid var(--error);
}

.info-box-icon {
    font-size: 1.2em;
    flex-shrink: 0;
}

.info-box-content h4 {
    font-size: 0.9em;
    margin-bottom: var(--spacing-xs);
}

.info-box-content p {
    font-size: 0.95em;
    color: rgba(240, 246, 252, 0.8);
    margin: 0;
}

.info-box code {
    background: rgba(255,255,255,0.1);
    padding: 2px 6px;
    border-radius: 4px;
    font-family: var(--font-mono);
    font-size: 0.9em;
}

/* ============================================
   CONCEPT CARDS
   ============================================ */

.concepts-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(250px, 1fr));
    gap: var(--spacing-md);
    margin: var(--spacing-lg) 0;
}

.concept-card {
    background: var(--code-surface);
    border-radius: 12px;
    padding: var(--spacing-lg);
    border: 1px solid rgba(255,255,255,0.05);
    transition: all var(--transition-normal);
}

.concept-card:hover {
    border-color: var(--chapter-primary);
    transform: translateY(-2px);
}

.concept-card h4 {
    display: flex;
    align-items: center;
    gap: var(--spacing-sm);
    color: var(--rust-light);
    margin-bottom: var(--spacing-sm);
}

.concept-card h4 code {
    background: rgba(var(--chapter-primary), 0.2);
    color: var(--chapter-primary);
    padding: 2px 8px;
    border-radius: 4px;
    font-size: 0.85em;
}

.concept-card p {
    font-size: 0.9em;
    color: var(--comment);
    margin: 0;
}

/* Type Grid */
.type-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(100px, 1fr));
    gap: var(--spacing-sm);
    margin: var(--spacing-lg) 0;
}

.type-cell {
    background: var(--code-surface);
    border-radius: 8px;
    padding: var(--spacing-md);
    text-align: center;
    border: 2px solid transparent;
    transition: all var(--transition-fast);
    cursor: pointer;
}

.type-cell:hover {
    border-color: var(--chapter-primary);
    transform: translateY(-2px);
}

.type-cell.selected {
    border-color: var(--success);
    background: var(--success-bg);
}

.type-cell .type-name {
    font-family: var(--font-mono);
    font-size: 1.1em;
    color: var(--type);
    margin-bottom: var(--spacing-xs);
}

.type-cell .type-desc {
    font-size: 0.75em;
    color: var(--comment);
}

/* Comparison Visuals */
.comparison-visual {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(200px, 1fr));
    gap: var(--spacing-lg);
    margin: var(--spacing-lg) 0;
}

.comparison-item {
    background: var(--code-surface);
    border-radius: 12px;
    padding: var(--spacing-lg);
    text-align: center;
}

.comparison-item.highlighted {
    border: 2px solid var(--chapter-primary);
    background: rgba(6, 182, 212, 0.1);
}

.comparison-item h4 {
    margin-bottom: var(--spacing-md);
    color: var(--comment);
}

.comparison-item.highlighted h4 {
    color: var(--chapter-primary);
}

.comparison-code {
    font-family: var(--font-mono);
    font-size: 0.85em;
    background: rgba(0,0,0,0.3);
    padding: var(--spacing-md);
    border-radius: 6px;
    text-align: left;
}

/* Struct Visual */
.struct-visual {
    background: var(--code-bg);
    border-radius: 12px;
    padding: var(--spacing-lg);
    margin: var(--spacing-lg) 0;
    border: 1px solid var(--code-surface);
}

.struct-visual-header {
    display: flex;
    align-items: center;
    justify-content: space-between;
    margin-bottom: var(--spacing-lg);
    flex-wrap: wrap;
    gap: var(--spacing-md);
}

.struct-visual-title {
    display: flex;
    align-items: center;
    gap: var(--spacing-sm);
    font-weight: 600;
    color: var(--comment);
}

.struct-box {
    background: var(--code-surface);
    border: 2px solid var(--chapter-primary);
    border-radius: 12px;
    overflow: hidden;
    min-width: 280px;
    margin: 0 auto;
}

.struct-name {
    background: rgba(6, 182, 212, 0.2);
    padding: var(--spacing-md);
    font-family: var(--font-mono);
    font-weight: 600;
    color: var(--type);
    text-align: center;
    border-bottom: 2px solid var(--chapter-primary);
}

.struct-fields {
    padding: var(--spacing-md);
}

.struct-field {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: var(--spacing-sm) var(--spacing-md);
    background: rgba(0,0,0,0.2);
    border-radius: 6px;
    margin-bottom: var(--spacing-sm);
    font-family: var(--font-mono);
    font-size: 0.9em;
}

.struct-field:last-child { margin-bottom: 0; }

.struct-field .field-name { color: #7dd3fc; }
.struct-field .field-type { color: var(--type); opacity: 0.7; }
.struct-field .field-value { color: var(--string); }

/* Enum Visual */
.enum-visual {
    background: var(--code-bg);
    border-radius: 12px;
    padding: var(--spacing-lg);
    margin: var(--spacing-lg) 0;
    border: 1px solid var(--code-surface);
}

.enum-visual-header {
    display: flex;
    align-items: center;
    justify-content: space-between;
    margin-bottom: var(--spacing-lg);
    flex-wrap: wrap;
    gap: var(--spacing-md);
}

.enum-visual-title {
    display: flex;
    align-items: center;
    gap: var(--spacing-sm);
    font-weight: 600;
    color: var(--comment);
}

.enum-box {
    background: var(--code-surface);
    border: 2px solid var(--chapter-primary);
    border-radius: 12px;
    overflow: hidden;
    min-width: 280px;
    margin: 0 auto;
}

.enum-name {
    background: rgba(244, 63, 94, 0.2);
    padding: var(--spacing-md);
    font-family: var(--font-mono);
    font-weight: 600;
    color: var(--type);
    text-align: center;
    border-bottom: 2px solid var(--chapter-primary);
}

.enum-variants {
    padding: var(--spacing-md);
}

.enum-variant {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: var(--spacing-sm) var(--spacing-md);
    background: rgba(0,0,0,0.2);
    border-radius: 6px;
    margin-bottom: var(--spacing-sm);
    font-family: var(--font-mono);
    font-size: 0.9em;
    cursor: pointer;
    transition: all var(--transition-fast);
    border: 2px solid transparent;
}

.enum-variant:last-child { margin-bottom: 0; }
.enum-variant:hover { background: rgba(244, 63, 94, 0.1); }
.enum-variant.selected {
    border-color: var(--chapter-primary);
    background: rgba(244, 63, 94, 0.2);
}

.enum-variant .variant-name { color: #c4b5fd; }
.enum-variant .variant-data { color: var(--type); opacity: 0.7; font-size: 0.85em; }

.variant-info {
    margin-top: var(--spacing-lg);
    text-align: center;
    font-family: var(--font-mono);
    color: var(--comment);
}

/* Option Visual */
.option-visual {
    display: flex;
    gap: var(--spacing-lg);
    justify-content: center;
    flex-wrap: wrap;
    margin: var(--spacing-lg) 0;
}

.option-box {
    background: var(--code-surface);
    border-radius: 12px;
    padding: var(--spacing-lg);
    min-width: 200px;
    text-align: center;
    border: 2px solid transparent;
    transition: all var(--transition-fast);
}

.option-box.some { border-color: var(--success); }
.option-box.none { border-color: var(--warning); }

.option-type {
    font-family: var(--font-mono);
    font-size: 0.85em;
    color: var(--type);
    margin-bottom: var(--spacing-sm);
}

.option-variant {
    font-family: var(--font-mono);
    font-weight: 600;
    font-size: 1.2em;
    margin-bottom: var(--spacing-sm);
}

.option-variant.some { color: var(--success); }
.option-variant.none { color: var(--warning); }

.option-value {
    font-family: var(--font-mono);
    color: var(--number);
}

/* Null Comparison */
.null-comparison {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(280px, 1fr));
    gap: var(--spacing-lg);
    margin: var(--spacing-lg) 0;
}

.lang-box {
    background: var(--code-bg);
    border-radius: 12px;
    overflow: hidden;
    border: 1px solid var(--code-surface);
}

.lang-header {
    padding: var(--spacing-md) var(--spacing-lg);
    background: var(--code-surface);
    font-weight: 600;
    display: flex;
    align-items: center;
    gap: var(--spacing-sm);
}

.lang-header.bad { color: var(--error); }
.lang-header.good { color: var(--success); }

.lang-code {
    padding: var(--spacing-lg);
    font-family: var(--font-mono);
    font-size: 0.85em;
    line-height: 1.8;
}

.lang-result {
    padding: var(--spacing-md) var(--spacing-lg);
    background: rgba(0,0,0,0.2);
    font-family: var(--font-mono);
    font-size: 0.85em;
}

.lang-result.crash { color: var(--error); }
.lang-result.safe { color: var(--success); }

/* Match Visual */
.match-visual {
    background: var(--code-bg);
    border-radius: 12px;
    padding: var(--spacing-lg);
    margin: var(--spacing-lg) 0;
    border: 1px solid var(--code-surface);
}

.match-header {
    display: flex;
    align-items: center;
    gap: var(--spacing-md);
    margin-bottom: var(--spacing-lg);
}

.match-input {
    display: flex;
    align-items: center;
    gap: var(--spacing-sm);
    font-family: var(--font-mono);
}

.match-input-label { color: var(--comment); }
.match-input-value {
    background: var(--code-surface);
    padding: var(--spacing-sm) var(--spacing-md);
    border-radius: 6px;
    color: #c4b5fd;
    border: 2px solid var(--chapter-primary);
}

.match-arms {
    display: flex;
    flex-direction: column;
    gap: var(--spacing-sm);
}

.match-arm {
    display: flex;
    align-items: center;
    gap: var(--spacing-md);
    padding: var(--spacing-md);
    background: rgba(0,0,0,0.2);
    border-radius: 8px;
    font-family: var(--font-mono);
    font-size: 0.9em;
    transition: all var(--transition-normal);
    border: 2px solid transparent;
}

.match-arm.checking {
    border-color: var(--warning);
    background: var(--warning-bg);
}

.match-arm.matched {
    border-color: var(--success);
    background: var(--success-bg);
}

.match-arm.skipped { opacity: 0.4; }

.arm-pattern { color: #c4b5fd; min-width: 120px; }
.arm-arrow { color: var(--comment); }
.arm-result { color: var(--number); }

.match-output {
    margin-top: var(--spacing-lg);
    padding: var(--spacing-md);
    background: var(--success-bg);
    border: 2px solid var(--success);
    border-radius: 8px;
    font-family: var(--font-mono);
    color: var(--success);
    text-align: center;
    opacity: 0;
    transition: opacity var(--transition-normal);
}

.match-output.visible { opacity: 1; }

/* Data Flow */
.data-flow {
    display: flex;
    align-items: center;
    justify-content: center;
    gap: var(--spacing-lg);
    flex-wrap: wrap;
    margin: var(--spacing-lg) 0;
}

.data-box {
    background: var(--code-surface);
    border: 2px solid var(--chapter-primary);
    border-radius: 8px;
    padding: var(--spacing-md) var(--spacing-lg);
    font-family: var(--font-mono);
    text-align: center;
}

.data-arrow {
    font-size: 1.5em;
    color: var(--chapter-primary);
}

/* Message Cards */
.message-variants {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(200px, 1fr));
    gap: var(--spacing-md);
    margin: var(--spacing-lg) 0;
}

.message-card {
    background: var(--code-surface);
    border-radius: 8px;
    padding: var(--spacing-lg);
    border: 2px solid transparent;
    transition: all var(--transition-fast);
    cursor: pointer;
}

.message-card:hover { border-color: var(--chapter-primary); }
.message-card.selected {
    border-color: var(--chapter-primary);
    background: rgba(244, 63, 94, 0.1);
}

.message-card-name {
    font-family: var(--font-mono);
    color: #c4b5fd;
    font-weight: 600;
    margin-bottom: var(--spacing-sm);
}

.message-card-data {
    font-family: var(--font-mono);
    font-size: 0.85em;
    color: var(--comment);
}

.message-card-desc {
    font-size: 0.85em;
    color: var(--comment);
    margin-top: var(--spacing-sm);
}

/* Warning Box */
.warning-box {
    background: var(--warning-bg);
    border-left: 4px solid var(--warning);
    padding: var(--spacing-lg);
    border-radius: 0 8px 8px 0;
    margin: var(--spacing-lg) 0;
}

.warning-box h4 {
    color: var(--warning);
    margin-bottom: var(--spacing-sm);
}

/* If Let Comparison */
.if-let-comparison {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
    gap: var(--spacing-lg);
    margin: var(--spacing-lg) 0;
}

.comparison-box {
    background: var(--code-bg);
    border-radius: 12px;
    overflow: hidden;
    border: 1px solid var(--code-surface);
}

.comparison-header {
    padding: var(--spacing-md) var(--spacing-lg);
    background: var(--code-surface);
    font-weight: 600;
    font-size: 0.9em;
}

/* Demo Container */
.demo-container {
    background: var(--code-bg);
    border-radius: 12px;
    padding: var(--spacing-lg);
    margin: var(--spacing-lg) 0;
    border: 1px solid var(--code-surface);
}

.demo-controls {
    display: flex;
    gap: var(--spacing-md);
    margin-bottom: var(--spacing-lg);
    flex-wrap: wrap;
}

.demo-btn {
    padding: var(--spacing-sm) var(--spacing-lg);
    background: var(--code-surface);
    border: 2px solid var(--chapter-primary);
    border-radius: 8px;
    color: var(--rust-light);
    font-family: var(--font-mono);
    font-size: 0.9em;
    cursor: pointer;
    transition: all var(--transition-fast);
}

.demo-btn:hover { background: var(--chapter-primary); }
.demo-btn.active { background: var(--chapter-primary); }

/* ============================================
   TERMINAL
   ============================================ */

.terminal {
    background: #000;
    border-radius: 12px;
    overflow: hidden;
    margin: var(--spacing-lg) 0;
    box-shadow: 0 8px 32px rgba(0,0,0,0.4);
}

.terminal-header {
    background: #2d2d2d;
    padding: var(--spacing-sm) var(--spacing-md);
    display: flex;
    align-items: center;
    gap: var(--spacing-sm);
}

.terminal-dot {
    width: 12px;
    height: 12px;
    border-radius: 50%;
}

.terminal-dot.red { background: #ff5f57; }
.terminal-dot.yellow { background: #febc2e; }
.terminal-dot.green { background: #28c840; }

.terminal-title {
    flex: 1;
    text-align: center;
    font-size: 0.8em;
    color: var(--comment);
}

.terminal-body {
    padding: var(--spacing-lg);
    font-family: var(--font-mono);
    font-size: 0.9em;
    min-height: 120px;
}

.terminal-line {
    margin-bottom: var(--spacing-xs);
    line-height: 1.6;
}

.terminal-prompt { color: var(--success); }
.terminal-command { color: var(--rust-light); }
.terminal-output { color: var(--comment); }
.terminal-error { color: var(--error); }

.terminal-input {
    display: flex;
    align-items: center;
    gap: var(--spacing-sm);
    margin-top: var(--spacing-md);
}

.terminal-input input {
    flex: 1;
    background: transparent;
    border: none;
    color: var(--rust-light);
    font-family: var(--font-mono);
    font-size: 0.9em;
    outline: none;
}

/* ============================================
   BUTTONS
   ============================================ */

.btn {
    display: inline-flex;
    align-items: center;
    justify-content: center;
    gap: var(--spacing-sm);
    padding: var(--spacing-md) var(--spacing-lg);
    border-radius: 8px;
    font-weight: 600;
    font-size: 0.95em;
    cursor: pointer;
    transition: all var(--transition-fast);
    border: none;
    text-decoration: none;
}

.btn-primary {
    background: var(--chapter-primary);
    color: white;
}

.btn-primary:hover {
    background: var(--chapter-primary-light);
    transform: translateY(-1px);
}

.btn-secondary {
    background: var(--code-surface);
    color: var(--rust-light);
    border: 1px solid rgba(255,255,255,0.1);
}

.btn-secondary:hover {
    background: rgba(255,255,255,0.1);
    border-color: rgba(255,255,255,0.2);
}

.btn-ghost {
    background: transparent;
    color: var(--comment);
}

.btn-ghost:hover {
    color: var(--rust-light);
    background: rgba(255,255,255,0.05);
}

.btn-group {
    display: flex;
    gap: var(--spacing-md);
    margin-top: var(--spacing-xl);
    flex-wrap: wrap;
}

/* ============================================
   INTERACTIVE PLAYGROUND
   ============================================ */

.playground {
    background: linear-gradient(135deg, var(--code-bg) 0%, var(--rust-darker) 100%);
    border-radius: 16px;
    padding: var(--spacing-xl);
    margin: var(--spacing-xl) 0;
    border: 2px solid var(--chapter-primary);
    text-align: center;
}

.playground-title {
    font-size: 1.8em;
    margin-bottom: var(--spacing-md);
    color: var(--chapter-primary);
}

.playground-subtitle {
    color: var(--comment);
    margin-bottom: var(--spacing-xl);
}

.game-input-wrapper {
    margin-bottom: var(--spacing-lg);
}

.game-input {
    width: 100%;
    max-width: 200px;
    padding: var(--spacing-md);
    font-size: 1.5em;
    text-align: center;
    background: var(--code-surface);
    border: 2px solid var(--chapter-primary);
    border-radius: 8px;
    color: var(--rust-light);
    font-family: var(--font-mono);
}

.game-input:focus {
    outline: none;
    border-color: var(--chapter-primary-light);
    box-shadow: 0 0 20px rgba(var(--chapter-primary), 0.3);
}

.game-controls {
    display: flex;
    gap: var(--spacing-md);
    justify-content: center;
    flex-wrap: wrap;
}

.guess-history {
    margin-top: var(--spacing-xl);
    text-align: left;
    background: var(--code-surface);
    border-radius: 8px;
    padding: var(--spacing-lg);
}

.guess-history h4 {
    font-size: 0.9em;
    color: var(--comment);
    margin-bottom: var(--spacing-md);
}

.guess-item {
    display: flex;
    align-items: center;
    gap: var(--spacing-md);
    padding: var(--spacing-sm) 0;
    border-bottom: 1px solid rgba(255,255,255,0.05);
}

.guess-number {
    font-family: var(--font-mono);
    font-size: 1.2em;
    min-width: 40px;
}

.guess-result {
    font-size: 0.9em;
}

.guess-result.high { color: var(--error); }
.guess-result.low { color: var(--info); }
.guess-result.correct { color: var(--success); }

/* ============================================
   FOOTER
   ============================================ */

.footer {
    text-align: center;
    padding: var(--spacing-2xl) var(--spacing-lg);
    border-top: 1px solid var(--code-surface);
    margin-top: var(--spacing-2xl);
}

.chapter-link {
    display: inline-flex;
    align-items: center;
    gap: var(--spacing-md);
    padding: var(--spacing-lg) var(--spacing-xl);
    background: var(--chapter-primary);
    color: white;
    text-decoration: none;
    border-radius: 12px;
    font-weight: 600;
    transition: all var(--transition-normal);
    margin-bottom: var(--spacing-xl);
    margin-right: var(--spacing-md);
}

.chapter-link:hover {
    background: var(--chapter-primary-dark);
    transform: translateY(-2px);
    box-shadow: 0 8px 24px rgba(var(--chapter-primary), 0.3);
}

.footer-text {
    color: var(--comment);
    font-size: 0.9em;
}

.footer-text a {
    color: var(--chapter-primary);
    text-decoration: none;
}

.footer-text a:hover {
    text-decoration: underline;
}

/* ============================================
   RESPONSIVE
   ============================================ */

@media (max-width: 768px) {
    .hero {
        padding: var(--spacing-xl) var(--spacing-md);
    }

    .hero h1 {
        font-size: 1.8em;
    }

    .nav-inner {
        flex-direction: column;
        gap: var(--spacing-md);
    }

    .progress-wrapper {
        width: 100%;
        max-width: none;
    }

    .code-body {
        flex-direction: column;
    }

    .line-numbers {
        display: none;
    }

    .concepts-grid {
        grid-template-columns: 1fr;
    }

    .comparison-visual {
        grid-template-columns: 1fr;
    }

    .null-comparison {
        grid-template-columns: 1fr;
    }

    .if-let-comparison {
        grid-template-columns: 1fr;
    }

    .message-variants {
        grid-template-columns: 1fr;
    }

    .btn-group {
        flex-direction: column;
    }

    .btn {
        width: 100%;
    }

    .game-controls {
        flex-direction: column;
    }
}