/* ============================================
   Tactile Sensing for Robot Hands — Web Viewer
   Dark-mode glassmorphism design
   ============================================ */

@import url('https://fonts.googleapis.com/css2?family=Inter:wght@300;400;500;700&family=Noto+Sans+KR:wght@300;400;500;700;900&display=swap');

/* --- CSS Variables --- */
:root {
  --bg-primary: #0a0a0f;
  --bg-secondary: #12121a;
  --bg-card: rgba(255,255,255,0.03);
  --bg-card-hover: rgba(255,255,255,0.06);
  --text-primary: #e0e0e0;
  --text-secondary: #a0a0a0;
  --text-muted: #666;
  --border-subtle: rgba(255,255,255,0.08);
  --border-hover: rgba(255,255,255,0.15);
  --accent-part1: #00D4AA;
  --accent-part2: #3498DB;
  --accent-part3: #9B59B6;
  --accent-part4: #FF6B35;
  --glow-part1: rgba(0, 212, 170, 0.15);
  --glow-part2: rgba(52, 152, 219, 0.15);
  --glow-part3: rgba(155, 89, 182, 0.15);
  --glow-part4: rgba(255, 107, 53, 0.15);
  --font-ko: 'Noto Sans KR', sans-serif;
  --font-en: 'Inter', 'Noto Sans KR', sans-serif;
  --radius-card: 16px;
  --radius-sm: 8px;
  --transition: all 0.3s ease;
  --max-width: 1200px;
  --content-width: 800px;
  --sidebar-width: 80px;
}

.part-1 { --accent: var(--accent-part1); --accent-glow: var(--glow-part1); }
.part-2 { --accent: var(--accent-part2); --accent-glow: var(--glow-part2); }
.part-3 { --accent: var(--accent-part3); --accent-glow: var(--glow-part3); }
.part-4 { --accent: var(--accent-part4); --accent-glow: var(--glow-part4); }

/* --- Reset --- */
*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }

html {
  scroll-behavior: smooth;
  scroll-padding-top: 100px;
  overflow-x: hidden;
}

body {
  font-family: var(--font-ko);
  background: var(--bg-primary);
  color: var(--text-primary);
  line-height: 1.8;
  font-weight: 400;
  overflow-x: hidden;
  overflow-wrap: break-word;
  word-break: break-word;
  min-height: 100vh;
  max-width: 100vw;
}

body.lang-en { font-family: var(--font-en); }

/* Gradient overlay */
body::before {
  content: '';
  position: fixed;
  top: 0; left: 0;
  width: 100%; height: 100%;
  background:
    radial-gradient(ellipse at 20% 50%, rgba(155, 89, 182, 0.08) 0%, transparent 50%),
    radial-gradient(ellipse at 80% 20%, rgba(52, 152, 219, 0.06) 0%, transparent 50%),
    radial-gradient(ellipse at 50% 80%, rgba(0, 212, 170, 0.05) 0%, transparent 50%);
  pointer-events: none;
  z-index: 0;
}

/* --- Particle Canvas --- */
#particle-canvas {
  position: fixed;
  top: 0; left: 0;
  width: 100%; height: 100%;
  z-index: 0;
  pointer-events: none;
}

/* --- Header --- */
.site-header {
  position: fixed;
  top: 0; left: 0; right: 0;
  z-index: 1000;
  background: rgba(10, 10, 15, 0.85);
  backdrop-filter: blur(20px);
  -webkit-backdrop-filter: blur(20px);
  border-bottom: 1px solid var(--border-subtle);
  padding: 0 2rem;
  height: 60px;
  display: flex;
  align-items: center;
  justify-content: space-between;
}

/* Header layout: icons left, title center, icons right */
.header-left { flex: 0 0 auto; }
.header-right { flex: 0 0 auto; }

.header-left {
  display: flex;
  align-items: center;
  gap: 1rem;
}

.header-logo {
  position: absolute;
  left: 50%;
  transform: translateX(-50%);
  font-family: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
  font-size: 0.95rem;
  font-weight: 500;
  letter-spacing: -0.01em;
  color: var(--text-primary);
  text-decoration: none;
  white-space: nowrap;
}

.header-logo:hover { color: #fff; }

.header-right {
  display: flex;
  align-items: center;
  gap: 0.8rem;
}

.header-icon {
  color: #ccc;
  text-decoration: none;
  font-size: 1.3rem;
  display: flex;
  align-items: center;
  justify-content: center;
  width: 36px;
  height: 36px;
  border-radius: 8px;
  background: rgba(255,255,255,0.06);
  border: 1px solid rgba(255,255,255,0.1);
  transition: all 0.3s;
}
.header-icon:hover {
  color: #fff;
  background: rgba(255,255,255,0.12);
  border-color: rgba(255,255,255,0.2);
}
.header-icon svg {
  width: 18px;
  height: 18px;
  fill: #ccc;
  transition: fill 0.3s;
}
.header-icon:hover svg { fill: #fff; }

.lang-toggle {
  display: flex;
  background: var(--bg-card);
  border: 1px solid var(--border-subtle);
  border-radius: 20px;
  overflow: hidden;
}

.lang-toggle a {
  padding: 0.3rem 0.8rem;
  font-size: 0.8rem;
  font-weight: 500;
  color: var(--text-secondary);
  text-decoration: none;
  transition: var(--transition);
}

.lang-toggle a.active {
  background: rgba(255,255,255,0.1);
  color: #fff;
}

.lang-toggle a:hover:not(.active) {
  color: var(--text-primary);
}

/* --- Main Content --- */
main {
  position: relative;
  z-index: 1;
  padding-top: 60px;
}

.container {
  max-width: var(--max-width);
  margin: 0 auto;
  padding: 0 2rem;
}

/* --- Hero Section --- */
.hero {
  min-height: 50vh;
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  text-align: center;
  padding: 3rem 2rem 1.5rem;
  position: relative;
}

.gradient-text {
  background: linear-gradient(135deg, #00D4AA, #3498DB, #9B59B6, #E8396B, #FF6B35);
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-clip: text;
  font-size: clamp(2rem, 5vw, 4rem);
  font-weight: 900;
  line-height: 1.2;
  margin-bottom: 1rem;
}

.hero .subtitle {
  font-size: clamp(1rem, 2vw, 1.4rem);
  color: var(--text-secondary);
  font-weight: 300;
  margin-bottom: 2rem;
}

.hero .description {
  max-width: 700px;
  font-size: 1rem;
  color: var(--text-muted);
  line-height: 1.8;
}

.hero-cta {
  margin-top: 2rem;
  display: flex;
  justify-content: center;
  gap: 1rem;
}

.hero-links {
  margin-top: 1rem;
  display: flex;
  justify-content: center;
  align-items: center;
  gap: 0.6rem;
  font-size: 0.85rem;
}

.hero-links a {
  color: var(--text-muted);
  text-decoration: none;
  transition: color 0.3s;
}

.hero-links a:hover {
  color: var(--accent-part1);
  text-decoration: underline;
}

.hero-links-sep {
  color: var(--border-subtle);
}

.btn-primary {
  display: inline-flex;
  align-items: center;
  gap: 0.5rem;
  padding: 0.8rem 2rem;
  background: linear-gradient(135deg, var(--accent-part1), var(--accent-part2));
  color: #fff;
  border: none;
  border-radius: 30px;
  font-size: 1rem;
  font-weight: 500;
  text-decoration: none;
  cursor: pointer;
  transition: var(--transition);
}

.btn-primary:hover {
  transform: translateY(-2px);
  box-shadow: 0 8px 30px rgba(0, 212, 170, 0.3);
}

.btn-secondary {
  display: inline-flex;
  align-items: center;
  gap: 0.5rem;
  padding: 0.8rem 2rem;
  background: transparent;
  color: var(--text-primary);
  border: 1px solid var(--border-hover);
  border-radius: 30px;
  font-size: 1rem;
  font-weight: 500;
  text-decoration: none;
  cursor: pointer;
  transition: var(--transition);
}

.btn-secondary:hover {
  transform: translateY(-2px);
  border-color: var(--accent-part1);
  color: var(--accent-part1);
  box-shadow: 0 8px 30px rgba(0, 212, 170, 0.15);
}

/* --- Intro List (flat, non-interactive) --- */
.intro-list {
  max-width: var(--content-width);
  margin: 0 auto;
  padding: 0 2rem;
}

.intro-item {
  display: flex;
  gap: 0.8rem;
  padding: 0.8rem 0;
  border-bottom: 1px solid var(--border-subtle);
}

.intro-item:last-child {
  border-bottom: none;
}

.intro-item .item-icon {
  flex-shrink: 0;
  font-size: 1.1rem;
  line-height: 1.4;
  opacity: 0.6;
}

.intro-item-content {
  flex: 1;
}

.intro-item h4 {
  font-size: 0.9rem;
  font-weight: 600;
  color: var(--text-primary);
  margin-bottom: 0.2rem;
}

.intro-item p {
  font-size: 0.82rem;
  color: var(--text-secondary);
  line-height: 1.5;
  margin: 0;
}

/* --- Intro Cards (legacy) --- */
.intro-section {
  padding: 2rem 0;
}

.intro-grid {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: 1.5rem;
  max-width: var(--max-width);
  margin: 0 auto;
  padding: 0 2rem;
}

.intro-card {
  background: var(--bg-card);
  border: 1px solid var(--border-subtle);
  border-radius: var(--radius-card);
  padding: 2rem;
  transition: var(--transition);
}

.intro-card:hover {
  border-color: var(--border-hover);
  background: var(--bg-card-hover);
  transform: translateY(-4px);
}

.intro-card .card-icon {
  font-size: 2rem;
  margin-bottom: 1rem;
  display: block;
}

.intro-card h3 {
  font-size: 1.2rem;
  font-weight: 700;
  margin-bottom: 0.75rem;
  color: var(--text-primary);
}

.intro-card p {
  font-size: 0.9rem;
  color: var(--text-secondary);
  line-height: 1.7;
}

/* --- Part / Chapter Grid --- */
.chapters-section {
  padding: 8rem 0 6rem;
}

.part-group {
  max-width: var(--max-width);
  margin: 0 auto 3rem;
  padding: 0 2rem;
}

.part-title {
  font-size: 1.3rem;
  font-weight: 700;
  margin-bottom: 1.5rem;
  padding-left: 1rem;
  border-left: 3px solid var(--accent);
  color: var(--text-primary);
}

.chapter-grid {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: 1.5rem;
}

.chapter-card {
  background: var(--bg-card);
  border: 1px solid var(--border-subtle);
  border-radius: var(--radius-card);
  padding: 1.8rem;
  text-decoration: none;
  color: inherit;
  transition: var(--transition);
  position: relative;
  overflow: hidden;
  display: flex;
  flex-direction: column;
}

.chapter-card::before {
  content: '';
  position: absolute;
  top: 0; left: 0; right: 0;
  height: 3px;
  background: var(--accent);
  opacity: 0;
  transition: opacity 0.3s;
}

.chapter-card:hover {
  transform: translateY(-6px);
  border-color: var(--accent);
  box-shadow: 0 12px 40px var(--accent-glow);
}

.chapter-card:hover::before { opacity: 1; }

.chapter-card .ch-num {
  font-size: 2rem;
  font-weight: 900;
  color: var(--accent);
  opacity: 0.6;
  margin-bottom: 0.5rem;
}

.chapter-card h3 {
  font-size: 1.05rem;
  font-weight: 600;
  margin-bottom: 0.5rem;
  color: var(--text-primary);
  line-height: 1.4;
}

.chapter-card p {
  font-size: 0.85rem;
  color: var(--text-secondary);
  line-height: 1.5;
  flex: 1;
}

.chapter-card .arrow {
  display: block;
  text-align: right;
  font-size: 1.2rem;
  color: var(--accent);
  opacity: 0;
  transition: opacity 0.3s;
  margin-top: 0.5rem;
}

.chapter-card:hover .arrow { opacity: 1; }

/* --- Chapter Page Layout --- */
.chapter-page {
  display: flex;
  min-height: 100vh;
}

.chapter-content {
  flex: 1;
  max-width: var(--content-width);
  width: 100%;
  margin: 0 auto;
  padding: 3rem 2rem 6rem;
  overflow-x: hidden;
  box-sizing: border-box;
}

/* Chapter Header */
.chapter-header {
  margin-bottom: 3rem;
  padding-bottom: 2rem;
  border-bottom: 1px solid var(--border-subtle);
}

.chapter-header .part-label {
  display: inline-block;
  font-size: 0.8rem;
  font-weight: 500;
  color: var(--accent);
  text-transform: uppercase;
  letter-spacing: 0.05em;
  margin-bottom: 0.5rem;
}

.chapter-header h1 {
  font-size: clamp(1.8rem, 4vw, 2.8rem);
  font-weight: 900;
  line-height: 1.3;
  margin-bottom: 1rem;
}

.chapter-meta {
  display: flex;
  gap: 1.5rem;
  font-size: 0.8rem;
  color: var(--text-muted);
  flex-wrap: wrap;
}

.chapter-meta span {
  display: flex;
  align-items: center;
  gap: 0.3rem;
}

.chapter-summary {
  margin-top: 1.5rem;
  font-size: 1rem;
  color: var(--text-secondary);
  line-height: 1.8;
}

/* Learning objectives box */
.learning-objectives {
  background: var(--bg-card);
  border: 1px solid var(--border-subtle);
  border-radius: var(--radius-card);
  padding: 1.5rem 2rem;
  margin: 1.5rem 0 2rem;
}

.learning-objectives strong {
  display: block;
  margin-bottom: 0.5rem;
  color: var(--text-primary);
}

.learning-objectives ul {
  list-style: none;
  padding: 0;
}

.learning-objectives li {
  padding: 0.3rem 0;
  padding-left: 1.5rem;
  position: relative;
  color: var(--text-secondary);
  font-size: 0.9rem;
}

.learning-objectives li::before {
  content: '\2713';
  position: absolute;
  left: 0;
  color: var(--accent);
}

/* --- Content Sections --- */
.content-section {
  margin-bottom: 3rem;
  opacity: 1;
  transform: translateY(0);
}

/* GSAP will handle animations via inline styles when loaded */

.content-section h2 {
  font-size: clamp(1.4rem, 2.5vw, 1.8rem);
  font-weight: 700;
  margin-bottom: 1.5rem;
  padding-top: 1rem;
  color: var(--text-primary);
}

.content-section h3 {
  font-size: clamp(1.1rem, 2vw, 1.4rem);
  font-weight: 600;
  margin: 2rem 0 1rem;
  color: var(--text-primary);
}

.content-section h4 {
  font-size: 1.1rem;
  font-weight: 600;
  margin: 1.5rem 0 0.75rem;
  color: var(--text-primary);
}

.content-section p {
  font-size: clamp(0.95rem, 1.2vw, 1.05rem);
  color: var(--text-secondary);
  line-height: 1.8;
  margin-bottom: 1rem;
}

.content-section ul, .content-section ol {
  margin: 1rem 0;
  padding-left: 1.5rem;
  color: var(--text-secondary);
}

.content-section li {
  margin-bottom: 0.5rem;
  line-height: 1.7;
  font-size: 0.95rem;
}

.content-section strong { color: var(--text-primary); }

.content-section a {
  color: var(--accent);
  text-decoration: none;
  border-bottom: 1px solid transparent;
  transition: border-color 0.3s;
}

.content-section a:hover {
  border-bottom-color: var(--accent);
}

/* --- Citation Superscripts --- */
sup a.cite-link {
  color: var(--accent);
  text-decoration: none;
  font-size: 0.8em;
  font-weight: 500;
  padding: 0 1px;
  border-bottom: none;
}
sup a.cite-link:hover {
  text-decoration: underline;
  border-bottom: none;
}

/* --- References List --- */
ol.references-list {
  list-style: none;
  counter-reset: ref-counter;
  padding-left: 0;
}
ol.references-list li {
  counter-increment: ref-counter;
  padding: 0.6rem 0.8rem;
  margin-bottom: 0.5rem;
  border-radius: 8px;
  font-size: 0.85rem;
  line-height: 1.6;
  color: var(--text-secondary);
  transition: background 0.3s, border-left 0.3s;
  border-left: 3px solid transparent;
}
ol.references-list li::before {
  content: "[" counter(ref-counter) "] ";
  color: var(--accent);
  font-weight: 600;
  font-size: 0.85rem;
}
ol.references-list li:target,
ol.references-list li.ref-highlight {
  background: var(--accent-glow);
  border-left: 3px solid var(--accent);
  padding-left: 12px;
}

/* Back-link arrow */
a.cite-backlink {
  color: var(--accent);
  text-decoration: none;
  font-size: 0.85rem;
  margin-left: 4px;
  opacity: 0.7;
  transition: opacity 0.2s;
}
a.cite-backlink:hover {
  opacity: 1;
  text-decoration: none;
}

/* Highlight citation when returning from reference */
.cite-back-highlight {
  background: var(--accent-glow);
  border-radius: 4px;
  padding: 0 4px;
  transition: background 0.3s;
}
ol.references-list li em {
  color: var(--text-muted);
}
ol.references-list li strong {
  color: var(--text-primary);
}
ol.references-list li a {
  word-break: break-all;
}

/* --- Key Paper Blockquote --- */
blockquote {
  background: rgba(52, 152, 219, 0.06);
  border-left: 3px solid var(--accent, var(--accent-part2));
  padding: 1rem 1.5rem;
  border-radius: 0 12px 12px 0;
  margin: 1.5rem 0;
  font-size: 0.9rem;
  color: var(--text-secondary);
  line-height: 1.7;
}

blockquote.key-paper {
  background: rgba(52, 152, 219, 0.08);
}

blockquote strong {
  color: var(--text-primary);
}

/* --- Tables --- */
.styled-table {
  width: 100%;
  border-collapse: collapse;
  margin: 1.5rem 0;
  font-size: 0.9rem;
  overflow-x: auto;
  display: block;
}

.styled-table thead {
  background: rgba(255, 255, 255, 0.05);
}

.styled-table th {
  padding: 0.8rem 1rem;
  text-align: left;
  font-weight: 600;
  color: var(--text-primary);
  border-bottom: 2px solid var(--border-subtle);
  white-space: nowrap;
  min-width: 80px;
}

.styled-table td {
  padding: 0.7rem 1rem;
  border-bottom: 1px solid var(--border-subtle);
  color: var(--text-secondary);
  word-break: break-word;
}

.styled-table td img {
  display: block;
  margin: 0 auto;
}

.styled-table td:has(img) {
  text-align: center;
}

.styled-table tbody tr:hover {
  background: rgba(255, 255, 255, 0.02);
}

/* --- Figures --- */
figure {
  margin: 2rem 0;
  text-align: center;
}

figure img {
  max-width: 100%;
  height: auto;
  border-radius: var(--radius-sm);
  border: 1px solid var(--border-subtle);
}

figure.placeholder-figure img {
  background: var(--bg-card);
  min-height: 200px;
}

figcaption {
  margin-top: 0.75rem;
  font-size: 0.85rem;
  color: var(--text-muted);
  line-height: 1.5;
}

/* --- Code Blocks --- */
pre {
  background: rgba(255, 255, 255, 0.03);
  border: 1px solid var(--border-subtle);
  border-radius: var(--radius-sm);
  padding: 1.2rem;
  overflow-x: auto;
  margin: 1.5rem 0;
  font-size: 0.85rem;
  line-height: 1.6;
}

code {
  font-family: 'JetBrains Mono', 'Fira Code', monospace;
  font-size: 0.85em;
  color: #e06c75;
  background: rgba(255, 255, 255, 0.05);
  padding: 0.15em 0.35em;
  border-radius: 4px;
}

pre code {
  color: var(--text-primary);
  background: none;
  padding: 0;
}

/* --- Sidebar Navigation --- */
.sidebar-nav {
  position: fixed;
  left: 2rem;
  top: 50%;
  transform: translateY(-50%);
  display: flex;
  flex-direction: column;
  gap: 1.2rem;
  z-index: 100;
}

.nav-dot {
  display: flex;
  align-items: center;
  gap: 0.75rem;
  cursor: pointer;
  text-decoration: none;
}

.nav-dot .dot {
  width: 10px;
  height: 10px;
  border-radius: 50%;
  background: rgba(255, 255, 255, 0.2);
  transition: var(--transition);
  flex-shrink: 0;
}

.nav-dot.active .dot {
  background: var(--accent);
  box-shadow: 0 0 10px var(--accent-glow);
  width: 12px;
  height: 12px;
}

.nav-dot .label {
  font-size: 0.75rem;
  color: var(--text-muted);
  opacity: 0.7;
  transition: opacity 0.3s;
  white-space: nowrap;
  max-width: 180px;
  overflow: hidden;
  text-overflow: ellipsis;
}

.sidebar-nav:hover .label { opacity: 1; }
.nav-dot.active .label { opacity: 1; color: var(--text-secondary); }

/* --- Chapter Navigation (prev/toc/next) --- */
.chapter-nav {
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding: 1.5rem 0;
  margin-top: 2rem;
  border-top: 1px solid var(--border-subtle);
}

.chapter-nav a {
  display: flex;
  align-items: center;
  gap: 0.5rem;
  padding: 0.6rem 1.2rem;
  background: var(--bg-card);
  border: 1px solid var(--border-subtle);
  border-radius: var(--radius-sm);
  color: var(--text-primary);
  text-decoration: none;
  font-size: 0.85rem;
  font-weight: 500;
  transition: var(--transition);
}

.chapter-nav a:hover {
  border-color: var(--accent);
  background: var(--bg-card-hover);
}

.chapter-nav .toc-link {
  color: var(--text-secondary);
  font-size: 0.8rem;
}

.chapter-nav .placeholder {
  visibility: hidden;
  width: 80px;
}

/* --- References Page --- */
.references-section {
  max-width: var(--content-width);
  margin: 0 auto;
  padding: 3rem 2rem 6rem;
}

.ref-filter {
  display: flex;
  flex-wrap: wrap;
  gap: 0.5rem;
  margin-bottom: 2rem;
}

.ref-filter button {
  padding: 0.4rem 1rem;
  background: var(--bg-card);
  border: 1px solid var(--border-subtle);
  border-radius: 20px;
  color: var(--text-secondary);
  font-size: 0.8rem;
  cursor: pointer;
  transition: var(--transition);
  font-family: inherit;
}

.ref-filter button.active,
.ref-filter button:hover {
  border-color: var(--accent, var(--accent-part2));
  color: var(--text-primary);
  background: rgba(52, 152, 219, 0.1);
}

.ref-item {
  padding: 1rem 0;
  border-bottom: 1px solid var(--border-subtle);
  font-size: 0.9rem;
  color: var(--text-secondary);
  line-height: 1.6;
}

.ref-item .ref-id {
  font-weight: 600;
  color: var(--accent, var(--accent-part2));
  margin-right: 0.5rem;
}

.ref-item a {
  color: var(--accent-part2);
  text-decoration: none;
}

.ref-item a:hover { text-decoration: underline; }

.ref-item .ref-chapters {
  display: inline-flex;
  gap: 0.3rem;
  margin-left: 0.5rem;
}

.ref-item .ref-chapters span {
  font-size: 0.7rem;
  padding: 0.1rem 0.4rem;
  background: rgba(255,255,255,0.05);
  border-radius: 4px;
  color: var(--text-muted);
}

/* --- Glossary Page --- */
.glossary-section {
  max-width: var(--content-width);
  margin: 0 auto;
  padding: 3rem 2rem 6rem;
}

.glossary-letter {
  font-size: 1.5rem;
  font-weight: 700;
  color: var(--accent-part2);
  margin: 2rem 0 1rem;
  padding-bottom: 0.5rem;
  border-bottom: 1px solid var(--border-subtle);
}

.glossary-item {
  padding: 0.6rem 0;
  font-size: 0.9rem;
  color: var(--text-secondary);
  line-height: 1.7;
}

.glossary-item strong {
  color: var(--text-primary);
}

/* --- Language Landing Page --- */
.lang-landing {
  min-height: 100vh;
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  text-align: center;
  gap: 3rem;
  padding: 2rem;
}

.lang-landing h1 {
  font-size: clamp(2rem, 5vw, 3.5rem);
  font-weight: 900;
}

.lang-options {
  display: flex;
  gap: 2rem;
}

.lang-option {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 1rem;
  padding: 2.5rem 3rem;
  background: var(--bg-card);
  border: 1px solid var(--border-subtle);
  border-radius: var(--radius-card);
  text-decoration: none;
  color: var(--text-primary);
  transition: var(--transition);
  min-width: 200px;
}

.lang-option:hover {
  transform: translateY(-6px);
  border-color: var(--accent-part2);
  box-shadow: 0 12px 40px var(--glow-part2);
}

.lang-option .lang-flag {
  font-size: 3rem;
}

.lang-option .lang-name {
  font-size: 1.2rem;
  font-weight: 600;
}

.lang-option .lang-sub {
  font-size: 0.85rem;
  color: var(--text-secondary);
}

/* --- Sources Section (bottom of chapter) --- */
.chapter-sources {
  margin-top: 3rem;
  padding-top: 2rem;
  border-top: 1px solid var(--border-subtle);
}

.chapter-sources h3 {
  font-size: 1.1rem;
  font-weight: 600;
  margin-bottom: 1rem;
  color: var(--text-primary);
}

.chapter-sources ul {
  list-style: none;
  padding: 0;
}

.chapter-sources li {
  padding: 0.3rem 0;
  font-size: 0.85rem;
  color: var(--text-muted);
  padding-left: 1rem;
  position: relative;
}

.chapter-sources li::before {
  content: '\2022';
  position: absolute;
  left: 0;
  color: var(--text-muted);
}

/* Chapter References Section */
.chapter-references {
  margin-top: 2rem;
  padding-top: 2rem;
  border-top: 1px solid var(--border-subtle);
}

.chapter-references h2 {
  font-size: 1.3rem;
  margin-bottom: 1rem;
}

.chapter-references ol {
  padding-left: 1.5rem;
}

.chapter-references li {
  padding: 0.4rem 0;
  font-size: 0.85rem;
  color: var(--text-secondary);
  line-height: 1.6;
}

/* --- Acknowledgment Section --- */
.acknowledgment-section {
  margin-top: 4rem;
  padding-top: 2rem;
  border-top: 1px solid var(--border-subtle);
}

.acknowledgment-section h2 {
  font-size: 1.2rem;
  font-weight: 600;
  color: var(--text-primary);
  margin-bottom: 1rem;
}

.acknowledgment-section p {
  font-size: 0.85rem;
  color: var(--text-secondary);
  line-height: 1.7;
  margin-bottom: 0.8rem;
}

.acknowledgment-section a {
  color: var(--accent-part1);
  text-decoration: none;
}

.acknowledgment-section a:hover {
  text-decoration: underline;
}

/* --- Footer --- */
.site-footer {
  position: relative;
  z-index: 1;
  text-align: center;
  padding: 2rem;
  border-top: 1px solid var(--border-subtle);
  font-size: 0.8rem;
  color: var(--text-muted);
}

.site-footer a {
  color: var(--text-secondary);
  text-decoration: none;
}

.site-footer a:hover { color: var(--text-primary); }

/* --- Responsive --- */
@media (max-width: 1100px) {
  .sidebar-nav { display: none; }
}

@media (max-width: 1024px) {
  .chapter-grid { grid-template-columns: repeat(2, 1fr); }
  .intro-grid { grid-template-columns: repeat(2, 1fr); }
}

@media (max-width: 768px) {
  .site-header { padding: 0 0.8rem; }
  .header-logo { font-size: 0.75rem; max-width: 60vw; overflow: hidden; text-overflow: ellipsis; }
  .chapter-grid { grid-template-columns: 1fr; }
  .intro-grid { grid-template-columns: 1fr; }
  .chapter-content { margin-left: 0; padding: 1.5rem 1rem 3rem; max-width: 100vw; }
  .content-section { overflow-x: hidden; }
  .content-section p,
  .content-section li { font-size: 0.9rem; line-height: 1.7; }
  .content-section h2 { font-size: 1.2rem; }
  .content-section h3 { font-size: 1.05rem; }
  .styled-table { font-size: 0.75rem; }
  .styled-table th { padding: 0.5rem; white-space: normal; min-width: 60px; }
  .styled-table td { padding: 0.5rem; }
  ol.references-list li { font-size: 0.8rem; padding: 0.4rem 0.5rem; }
  .hero { min-height: 60vh; padding: 3rem 1rem; }
  .lang-options { flex-direction: column; }
  .container { padding: 0 1rem; }
  .part-group { padding: 0 1rem; }
  .chapter-nav { flex-wrap: wrap; gap: 0.5rem; justify-content: center; }
  .chapter-nav a { flex: 0 0 auto; font-size: 0.8rem; padding: 0.5rem 0.8rem; }
  blockquote { padding: 0.6rem 1rem; font-size: 0.85rem; }
  figure img { max-width: 100%; }
  pre { font-size: 0.75rem; padding: 0.8rem; }
  .chapter-header h1 { font-size: 1.3rem; }
  .references-section,
  .glossary-section { padding: 2rem 1rem 4rem; }
}

@media (max-width: 480px) {
  .gradient-text { font-size: 1.4rem; }
  .chapter-card { padding: 1rem; }
  .intro-card { padding: 1rem; }
  .header-logo { font-size: 0.7rem; }
  .header-icon { width: 30px; height: 30px; }
  .chapter-content { padding: 1.2rem 0.8rem 3rem; }
  .content-section p,
  .content-section li { font-size: 0.85rem; }
}

/* --- Scrollbar --- */
::-webkit-scrollbar { width: 6px; }
::-webkit-scrollbar-track { background: var(--bg-primary); }
::-webkit-scrollbar-thumb { background: rgba(255,255,255,0.1); border-radius: 3px; }
::-webkit-scrollbar-thumb:hover { background: rgba(255,255,255,0.2); }

/* --- Selection --- */
::selection {
  background: rgba(52, 152, 219, 0.3);
  color: #fff;
}

/* --- Animation utilities --- */
.fade-in {
  opacity: 0;
  transform: translateY(20px);
  transition: opacity 0.6s ease, transform 0.6s ease;
}

.fade-in.visible {
  opacity: 1;
  transform: translateY(0);
}

/* --- Horizontal rule --- */
hr {
  border: none;
  border-top: 1px solid var(--border-subtle);
  margin: 2rem 0;
}