/* ========== CSS Variables ========== */
:root {
  --bg-deep: #0a0e1a;
  --bg-primary: #0f1525;
  --bg-card: #151d30;
  --bg-card-hover: #1a2540;
  --bg-sidebar: #0c1120;
  --border: #1e2a45;
  --border-active: #3b82f6;
  --text-primary: #e8ecf4;
  --text-secondary: #8b95ad;
  --text-muted: #5a6480;
  --accent-blue: #3b82f6;
  --accent-cyan: #06b6d4;
  --accent-purple: #8b5cf6;
  --accent-green: #10b981;
  --accent-orange: #f59e0b;
  --accent-red: #ef4444;
  --accent-pink: #ec4899;
  --gradient-1: linear-gradient(135deg, #3b82f6, #06b6d4);
  --gradient-2: linear-gradient(135deg, #8b5cf6, #ec4899);
  --gradient-3: linear-gradient(135deg, #10b981, #06b6d4);
  --shadow-sm: 0 1px 3px rgba(0,0,0,0.3);
  --shadow-md: 0 4px 12px rgba(0,0,0,0.4);
  --shadow-lg: 0 8px 30px rgba(0,0,0,0.5);
  --radius-sm: 8px;
  --radius-md: 12px;
  --radius-lg: 16px;
  --sidebar-width: 280px;
  --font-serif: 'Noto Serif SC', 'SimSun', serif;
  --font-sans: 'Noto Sans SC', 'Microsoft YaHei', sans-serif;
}

/* ========== Reset & Base ========== */
*, *::before, *::after { margin: 0; padding: 0; box-sizing: border-box; }
html { scroll-behavior: smooth; scrollbar-width: thin; scrollbar-color: var(--border) var(--bg-deep); }
body {
  font-family: var(--font-sans);
  background: var(--bg-deep);
  color: var(--text-primary);
  line-height: 1.7;
  overflow-x: hidden;
}
::-webkit-scrollbar { width: 6px; }
::-webkit-scrollbar-track { background: var(--bg-deep); }
::-webkit-scrollbar-thumb { background: var(--border); border-radius: 3px; }
::-webkit-scrollbar-thumb:hover { background: var(--text-muted); }
a { color: var(--accent-cyan); text-decoration: none; }
a:hover { color: var(--accent-blue); }

/* ========== Background Effects ========== */
.bg-glow {
  position: fixed; top: 0; left: 0; width: 100%; height: 100%;
  pointer-events: none; z-index: 0; overflow: hidden;
}
.bg-glow::before {
  content: ''; position: absolute; top: -20%; right: -10%;
  width: 600px; height: 600px;
  background: radial-gradient(circle, rgba(59,130,246,0.08) 0%, transparent 70%);
  animation: floatGlow 20s ease-in-out infinite;
}
.bg-glow::after {
  content: ''; position: absolute; bottom: -10%; left: -5%;
  width: 500px; height: 500px;
  background: radial-gradient(circle, rgba(139,92,246,0.06) 0%, transparent 70%);
  animation: floatGlow 25s ease-in-out infinite reverse;
}
@keyframes floatGlow {
  0%, 100% { transform: translate(0, 0); }
  50% { transform: translate(40px, -30px); }
}

/* ========== Layout ========== */
.app-container { display: flex; min-height: 100vh; position: relative; z-index: 1; }

/* ========== Sidebar ========== */
.sidebar {
  width: var(--sidebar-width); min-width: var(--sidebar-width);
  background: var(--bg-sidebar); border-right: 1px solid var(--border);
  height: 100vh; position: sticky; top: 0; display: flex;
  flex-direction: column; overflow: hidden; z-index: 100;
}
.sidebar-header {
  padding: 24px 20px 16px; border-bottom: 1px solid var(--border); flex-shrink: 0;
}
.sidebar-logo { display: flex; align-items: center; gap: 10px; margin-bottom: 6px; }
.sidebar-logo .icon-box {
  width: 36px; height: 36px; background: var(--gradient-1); border-radius: 8px;
  display: flex; align-items: center; justify-content: center; font-size: 16px; flex-shrink: 0;
}
.sidebar-logo h1 {
  font-family: var(--font-serif); font-size: 16px; font-weight: 700; color: var(--text-primary); line-height: 1.3;
}
.sidebar-logo .subtitle { font-size: 11px; color: var(--text-muted); letter-spacing: 1px; }

.sidebar-back {
  display: flex; align-items: center; gap: 8px; padding: 12px 20px;
  border-bottom: 1px solid var(--border); flex-shrink: 0;
}
.sidebar-back a {
  color: var(--text-secondary); font-size: 13px; display: flex; align-items: center; gap: 6px;
  transition: color 0.2s;
}
.sidebar-back a:hover { color: var(--accent-blue); }

.sidebar-nav { flex: 1; overflow-y: auto; padding: 12px; }
.nav-section { margin-bottom: 4px; }
.nav-section-title {
  padding: 8px 12px; font-size: 11px; font-weight: 600; color: var(--text-muted);
  text-transform: uppercase; letter-spacing: 1px;
}
.nav-link {
  display: block; padding: 8px 12px; font-size: 13px; color: var(--text-secondary);
  border-radius: 6px; transition: all 0.15s; text-decoration: none;
}
.nav-link:hover { color: var(--text-primary); background: rgba(59,130,246,0.05); }
.nav-link.active { color: var(--accent-blue); background: rgba(59,130,246,0.1); font-weight: 500; }
.nav-link .nav-badge {
  float: right; font-size: 10px; padding: 1px 6px; border-radius: 8px; font-weight: 500;
}
.nav-link .nav-badge.core { background: rgba(239,68,68,0.15); color: var(--accent-red); }
.nav-link .nav-badge.high { background: rgba(245,158,11,0.15); color: var(--accent-orange); }
.nav-link .nav-badge.mid { background: rgba(16,185,129,0.15); color: var(--accent-green); }

/* Sidebar progress */
.sidebar-progress { padding: 12px 20px; border-bottom: 1px solid var(--border); flex-shrink: 0; }
.progress-label { display: flex; justify-content: space-between; font-size: 11px; color: var(--text-muted); margin-bottom: 6px; }
.progress-bar { height: 4px; background: var(--bg-card); border-radius: 2px; overflow: hidden; }
.progress-track { height: 4px; background: var(--bg-card); border-radius: 2px; overflow: hidden; }
.progress-fill { height: 100%; background: var(--gradient-1); border-radius: 2px; transition: width 0.5s ease; }

/* Homepage-style sidebar nav */
.nav-list { list-style: none; padding: 0; }
.nav-link { display: flex; align-items: center; gap: 10px; padding: 9px 12px; font-size: 13px; color: var(--text-secondary); text-decoration: none; border-radius: var(--radius-sm); transition: all 0.2s; }
.nav-link:hover { color: var(--text-primary); background: rgba(59,130,246,0.06); }
.nav-link.active { color: var(--accent-blue); background: rgba(59,130,246,0.1); font-weight: 500; }
.nav-icon { width: 20px; display: flex; align-items: center; justify-content: center; flex-shrink: 0; color: var(--text-muted); }
.nav-link.active .nav-icon { color: var(--accent-blue); }
.nav-text { flex: 1; line-height: 1.4; }
.nav-section-label { padding: 14px 12px 6px; font-size: 11px; font-weight: 600; color: var(--text-muted); text-transform: uppercase; letter-spacing: 1px; }

/* Badges */
.badge { font-size: 10px; padding: 2px 6px; border-radius: 4px; font-weight: 600; flex-shrink: 0; line-height: 1.4; }
.badge-core { background: rgba(239,68,68,0.12); color: var(--accent-red); border: 1px solid rgba(239,68,68,0.2); }
.badge-high { background: rgba(245,158,11,0.12); color: var(--accent-orange); border: 1px solid rgba(245,158,11,0.2); }
.badge-mid { background: rgba(6,182,212,0.12); color: var(--accent-cyan); border: 1px solid rgba(6,182,212,0.2); }

/* Sidebar footer */
.sidebar-footer { padding: 16px 20px; border-top: 1px solid var(--border); text-align: center; font-size: 11px; color: var(--text-muted); flex-shrink: 0; }
.sidebar-footer .version { font-size: 10px; margin-top: 2px; opacity: 0.6; }

/* Toggle bar (hamburger icon) */
.toggle-bar { width: 18px; height: 2px; background: white; border-radius: 1px; transition: all 0.3s; }

/* ========== Main Content ========== */
.main-content { flex: 1; min-width: 0; }

/* ========== Hero Section ========== */
.hero { padding: 48px 48px 32px; position: relative; overflow: hidden; }
.hero::before {
  content: ''; position: absolute; top: 0; left: 0; right: 0; height: 100%;
  background: linear-gradient(180deg, rgba(59,130,246,0.04) 0%, transparent 100%);
  pointer-events: none;
}
.hero-content { position: relative; z-index: 1; }
.hero-badge {
  display: inline-flex; align-items: center; gap: 6px; padding: 5px 14px;
  background: rgba(59,130,246,0.1); border: 1px solid rgba(59,130,246,0.2);
  border-radius: 20px; font-size: 12px; color: var(--accent-blue); margin-bottom: 16px;
}
.hero h1 {
  font-family: var(--font-serif); font-size: 32px; font-weight: 900; line-height: 1.25;
  margin-bottom: 12px; background: linear-gradient(135deg, #e8ecf4, #8b95ad);
  -webkit-background-clip: text; -webkit-text-fill-color: transparent; background-clip: text;
}
.hero p { font-size: 14px; color: var(--text-secondary); max-width: 560px; line-height: 1.8; }

/* ========== Chapter Hero ========== */
.chapter-hero {
  padding: 40px 48px 24px; position: relative; overflow: hidden;
}
.chapter-hero::before {
  content: ''; position: absolute; top: 0; left: 0; right: 0; height: 100%;
  background: linear-gradient(180deg, rgba(59,130,246,0.04) 0%, transparent 100%);
  pointer-events: none;
}
.chapter-hero .chapter-badge {
  display: inline-flex; align-items: center; gap: 6px; padding: 4px 12px;
  background: rgba(59,130,246,0.1); border: 1px solid rgba(59,130,246,0.2);
  border-radius: 20px; font-size: 11px; color: var(--accent-blue); margin-bottom: 14px;
}
.chapter-hero h1 {
  font-family: var(--font-serif); font-size: 28px; font-weight: 800; margin-bottom: 10px;
  color: var(--text-primary);
}
.chapter-hero .chapter-meta {
  display: flex; gap: 24px; flex-wrap: wrap; margin-top: 12px;
}
.chapter-hero .meta-item {
  display: flex; align-items: center; gap: 6px; font-size: 12px; color: var(--text-secondary);
}
.chapter-hero .meta-item .meta-icon { font-size: 14px; }

/* ========== Content Area ========== */
.content-area { padding: 0 48px 60px; }

/* ========== Sections ========== */
.content-section { margin-bottom: 40px; animation: fadeInUp 0.4s ease forwards; opacity: 0; }
@keyframes fadeInUp {
  from { opacity: 0; transform: translateY(12px); }
  to { opacity: 1; transform: translateY(0); }
}

.section-title {
  display: flex; align-items: center; gap: 12px; margin-bottom: 20px;
  padding-bottom: 12px; border-bottom: 1px solid var(--border);
}
.section-title .section-icon {
  width: 32px; height: 32px; border-radius: 8px; display: flex; align-items: center;
  justify-content: center; font-size: 15px; flex-shrink: 0;
}
.section-title h2 { font-family: var(--font-serif); font-size: 18px; font-weight: 700; }
.section-title .weight-tag {
  margin-left: auto; font-size: 11px; padding: 3px 10px; border-radius: 12px; font-weight: 500;
}

/* ========== Knowledge Cards ========== */
.k-grid { display: grid; grid-template-columns: repeat(auto-fill, minmax(320px, 1fr)); gap: 14px; }

.k-card {
  background: var(--bg-card); border: 1px solid var(--border); border-radius: var(--radius-md);
  padding: 18px; transition: all 0.25s ease; cursor: default; position: relative; overflow: hidden;
}
.k-card::before {
  content: ''; position: absolute; top: 0; left: 0; width: 3px; height: 100%;
  border-radius: 0 2px 2px 0; opacity: 0; transition: opacity 0.2s;
}
.k-card:hover { border-color: rgba(59,130,246,0.3); background: var(--bg-card-hover); transform: translateY(-2px); box-shadow: var(--shadow-md); }
.k-card:hover::before { opacity: 1; }
.k-card.card-blue::before { background: var(--accent-blue); }
.k-card.card-cyan::before { background: var(--accent-cyan); }
.k-card.card-purple::before { background: var(--accent-purple); }
.k-card.card-green::before { background: var(--accent-green); }
.k-card.card-orange::before { background: var(--accent-orange); }
.k-card.card-red::before { background: var(--accent-red); }
.k-card.card-pink::before { background: var(--accent-pink); }

.k-card-title { font-size: 14px; font-weight: 600; margin-bottom: 8px; display: flex; align-items: center; gap: 6px; }
.k-card-title .tag { font-size: 10px; padding: 2px 7px; border-radius: 4px; font-weight: 500; flex-shrink: 0; }
.k-card-body { font-size: 13px; color: var(--text-secondary); line-height: 1.8; }
.k-card-body ul { list-style: none; padding: 0; }
.k-card-body ul li { padding: 3px 0 3px 16px; position: relative; }
.k-card-body ul li::before { content: '▸'; position: absolute; left: 0; color: var(--accent-cyan); font-size: 11px; }

/* ========== Chapter Top Nav ========== */
.chapter-nav {
  display: flex; gap: 8px; padding: 12px 48px; background: rgba(59,130,246,0.03);
  border-bottom: 1px solid var(--border); flex-wrap: wrap;
}
.chapter-nav-item {
  padding: 6px 14px; font-size: 12px; color: var(--text-secondary);
  border-radius: 6px; transition: all 0.15s; text-decoration: none;
}
.chapter-nav-item:hover { color: var(--text-primary); background: rgba(59,130,246,0.06); }
.chapter-nav-item.active { color: var(--accent-blue); background: rgba(59,130,246,0.1); font-weight: 500; }

/* ========== Section Aliases (HTML uses .section/.section-header) ========== */
.section { margin-bottom: 40px; animation: fadeInUp 0.4s ease forwards; opacity: 0; }
.section-header {
  display: flex; align-items: center; gap: 12px; margin-bottom: 20px;
  padding-bottom: 12px; border-bottom: 1px solid var(--border);
}
.section-header .section-icon {
  width: 32px; height: 32px; border-radius: 8px; display: flex; align-items: center;
  justify-content: center; font-size: 15px; flex-shrink: 0;
}
.section-header h2 { font-family: var(--font-serif); font-size: 18px; font-weight: 700; }
.section-header .section-tag {
  margin-left: auto; font-size: 11px; padding: 3px 10px; border-radius: 12px; font-weight: 500;
}

/* ========== Hero Stats Alias ========== */
.hero-stats { display: flex; gap: 28px; margin-top: 24px; flex-wrap: wrap; }
.stat-item { display: flex; flex-direction: column; }
.stat-item .value {
  font-size: 24px; font-weight: 700; font-family: var(--font-serif);
  background: var(--gradient-1); -webkit-background-clip: text;
  -webkit-text-fill-color: transparent; background-clip: text;
}
.stat-item .label { font-size: 11px; color: var(--text-muted); margin-top: 2px; }

/* ========== Knowledge Block ========== */
.k-block {
  background: var(--bg-card); border: 1px solid var(--border); border-radius: var(--radius-md);
  padding: 20px; margin-bottom: 14px; transition: all 0.25s ease;
}
.k-block:hover { border-color: rgba(59,130,246,0.15); }
.k-block h3 { font-size: 15px; font-weight: 600; margin-bottom: 12px; display: flex; align-items: center; gap: 8px; }
.k-block h3 .emoji { font-size: 15px; }

.k-block-grid { display: grid; grid-template-columns: repeat(auto-fill, minmax(260px, 1fr)); gap: 10px; }

.k-point {
  padding: 10px 12px; background: rgba(255,255,255,0.02); border-radius: var(--radius-sm);
  border-left: 2px solid var(--border); font-size: 13px; color: var(--text-secondary); line-height: 1.7;
  transition: all 0.2s;
}
.k-point:hover { background: rgba(59,130,246,0.04); border-left-color: var(--accent-blue); }
.k-point strong { color: var(--text-primary); font-weight: 600; }
.k-point.highlight { border-left-color: var(--accent-orange); background: rgba(245,158,11,0.04); }

/* ========== Exam Tip ========== */
.exam-tip {
  display: flex; gap: 12px; padding: 14px 16px; border-radius: var(--radius-sm);
  margin: 16px 0; font-size: 13px; line-height: 1.7;
}
.exam-tip.info { background: rgba(59,130,246,0.06); border: 1px solid rgba(59,130,246,0.15); }
.exam-tip.warn { background: rgba(245,158,11,0.06); border: 1px solid rgba(245,158,11,0.15); }
.exam-tip.danger { background: rgba(239,68,68,0.06); border: 1px solid rgba(239,68,68,0.15); }
.exam-tip.success { background: rgba(16,185,129,0.06); border: 1px solid rgba(16,185,129,0.15); }
.exam-tip .tip-icon { font-size: 15px; flex-shrink: 0; margin-top: 1px; }
.exam-tip code {
  background: rgba(59,130,246,0.1); padding: 1px 6px; border-radius: 3px;
  font-size: 12px; color: var(--accent-cyan);
}

/* ========== Table ========== */
.table-wrap { overflow-x: auto; margin-top: 12px; }
.k-table { width: 100%; border-collapse: collapse; font-size: 13px; }
.k-table th {
  background: rgba(59,130,246,0.08); padding: 10px 12px; text-align: left;
  font-weight: 600; font-size: 12px; color: var(--accent-blue); border-bottom: 1px solid var(--border);
  white-space: nowrap;
}
.k-table td { padding: 10px 12px; border-bottom: 1px solid rgba(30,42,69,0.5); color: var(--text-secondary); }
.k-table tr:hover td { background: rgba(59,130,246,0.03); }

/* ========== Comparison Cards ========== */
.compare-grid { display: grid; grid-template-columns: repeat(auto-fill, minmax(280px, 1fr)); gap: 14px; }
.compare-card {
  background: var(--bg-card); border: 1px solid var(--border); border-radius: var(--radius-md);
  padding: 18px; transition: all 0.2s;
}
.compare-card:hover { transform: translateY(-2px); box-shadow: var(--shadow-md); }
.compare-card h4 { font-size: 14px; font-weight: 600; margin-bottom: 8px; display: flex; align-items: center; gap: 6px; }
.compare-card .pros { color: var(--accent-green); }
.compare-card .cons { color: var(--accent-red); }
.compare-card .list { font-size: 13px; color: var(--text-secondary); line-height: 1.7; margin-top: 6px; }
.compare-card .list div { padding: 2px 0 2px 14px; position: relative; }
.compare-card .list div::before { content: '•'; position: absolute; left: 0; font-size: 14px; }
.compare-card .list.pros div::before { color: var(--accent-green); }
.compare-card .list.cons div::before { color: var(--accent-red); }

/* ========== Quiz Panel ========== */
.quiz-panel {
  background: var(--bg-card); border: 1px solid var(--border); border-radius: var(--radius-md);
  padding: 20px; margin-top: 20px;
}
.quiz-header { display: flex; align-items: center; justify-content: space-between; margin-bottom: 16px; }
.quiz-header h3 { font-size: 16px; font-weight: 700; display: flex; align-items: center; gap: 8px; }
.quiz-score { font-size: 14px; color: var(--accent-green); font-weight: 600; }

.quiz-question { margin-bottom: 20px; padding-bottom: 20px; border-bottom: 1px solid var(--border); }
.quiz-question:last-child { border-bottom: none; padding-bottom: 0; margin-bottom: 0; }
.quiz-q { font-size: 14px; font-weight: 600; margin-bottom: 12px; line-height: 1.6; }
.quiz-q .q-num { color: var(--accent-blue); }

.quiz-options { display: flex; flex-direction: column; gap: 8px; }
.quiz-option {
  display: flex; align-items: center; gap: 10px; padding: 10px 14px;
  background: rgba(255,255,255,0.02); border: 1px solid var(--border);
  border-radius: var(--radius-sm); cursor: pointer; transition: all 0.2s; font-size: 13px;
}
.quiz-option:hover { background: rgba(59,130,246,0.06); border-color: rgba(59,130,246,0.2); }
.quiz-option.selected { border-color: var(--accent-blue); background: rgba(59,130,246,0.1); }
.quiz-option.correct { border-color: var(--accent-green) !important; background: rgba(16,185,129,0.1) !important; }
.quiz-option.wrong { border-color: var(--accent-red) !important; background: rgba(239,68,68,0.1) !important; }
.quiz-option .opt-label {
  width: 24px; height: 24px; border-radius: 50%; border: 1px solid var(--border);
  display: flex; align-items: center; justify-content: center; font-size: 11px;
  font-weight: 600; flex-shrink: 0;
}
.quiz-option.correct .opt-label { border-color: var(--accent-green); background: var(--accent-green); color: white; }
.quiz-option.wrong .opt-label { border-color: var(--accent-red); background: var(--accent-red); color: white; }

.quiz-explanation {
  margin-top: 10px; padding: 10px 14px; background: rgba(59,130,246,0.04);
  border-radius: var(--radius-sm); font-size: 12px; color: var(--text-secondary); line-height: 1.7;
  display: none;
}
.quiz-explanation.show { display: block; }

.quiz-actions { display: flex; gap: 10px; margin-top: 16px; }
.btn {
  padding: 8px 20px; border: none; border-radius: var(--radius-sm); font-size: 13px;
  font-weight: 500; cursor: pointer; transition: all 0.2s; font-family: var(--font-sans);
}
.btn-primary { background: var(--gradient-1); color: white; }
.btn-primary:hover { opacity: 0.9; }
.btn-secondary { background: var(--bg-card-hover); color: var(--text-secondary); border: 1px solid var(--border); }
.btn-secondary:hover { border-color: var(--accent-blue); color: var(--text-primary); }

/* ========== Footer ========== */
.footer {
  padding: 24px 48px; border-top: 1px solid var(--border); text-align: center;
  font-size: 12px; color: var(--text-muted);
}

/* ========== Sidebar Nav Aliases (HTML uses .nav-group/.nav-item) ========== */
.nav-group { margin-bottom: 4px; }
.nav-group-header {
  display: flex; align-items: center; justify-content: space-between;
  padding: 8px 12px; cursor: pointer; border-radius: 6px;
  transition: all 0.2s; user-select: none;
}
.nav-group-header:hover { background: rgba(59,130,246,0.06); }
.nav-group-header.active { background: rgba(59,130,246,0.1); }
.nav-group-header .label {
  display: flex; align-items: center; gap: 8px;
  font-size: 13px; font-weight: 600; color: var(--text-primary);
}
.nav-group-header .label .dot { width: 8px; height: 8px; border-radius: 50%; flex-shrink: 0; }
.nav-group-header .arrow { font-size: 10px; color: var(--text-muted); transition: transform 0.2s; }
.nav-group-header.active .arrow { transform: rotate(90deg); }
.nav-items { max-height: 0; overflow: hidden; transition: max-height 0.3s ease; }
.nav-items.open { max-height: 600px; }
.nav-item {
  display: block; padding: 7px 12px; font-size: 12.5px; color: var(--text-secondary);
  border-radius: 6px; transition: all 0.15s; text-decoration: none; cursor: pointer; line-height: 1.5;
}
.nav-item:hover { color: var(--text-primary); background: rgba(59,130,246,0.05); }
.nav-item.active { color: var(--accent-blue); background: rgba(59,130,246,0.1); font-weight: 500; }

/* ========== Sidebar Search ========== */
.sidebar-search { padding: 12px 16px; }
.search-box {
  display: flex; align-items: center; background: var(--bg-card);
  border: 1px solid var(--border); border-radius: var(--radius-sm);
  padding: 0 10px; transition: border-color 0.2s;
}
.search-box:focus-within { border-color: var(--accent-blue); }
.search-box svg { color: var(--text-muted); flex-shrink: 0; }
.search-box input {
  flex: 1; background: none; border: none; outline: none;
  color: var(--text-primary); font-size: 13px; padding: 8px 8px; font-family: var(--font-sans);
}
.search-box input::placeholder { color: var(--text-muted); }

/* ========== Sidebar Progress Alias ========== */
.progress-bar-wrap { padding: 12px 20px; border-bottom: 1px solid var(--border); flex-shrink: 0; }

/* ========== Back to Top ========== */
.back-top {
  position: fixed; bottom: 24px; right: 84px; width: 40px; height: 40px;
  background: var(--bg-card); border: 1px solid var(--border); border-radius: 50%;
  color: var(--text-secondary); font-size: 16px; cursor: pointer;
  display: flex; align-items: center; justify-content: center;
  opacity: 0; transition: all 0.3s; z-index: 50;
}
.back-top.visible { opacity: 1; }
.back-top:hover { background: var(--bg-card-hover); color: var(--accent-blue); }

/* ========== Mobile ========== */
.mobile-toggle {
  display: none; position: fixed; bottom: 20px; right: 20px;
  width: 48px; height: 48px; background: var(--gradient-1); border: none;
  border-radius: 50%; color: white; font-size: 20px; cursor: pointer;
  box-shadow: var(--shadow-lg); z-index: 200; align-items: center; justify-content: center;
}
.sidebar-overlay { display: none; position: fixed; inset: 0; background: rgba(0,0,0,0.5); z-index: 90; }

/* ========== Responsive ========== */
@media (max-width: 900px) {
  .sidebar { position: fixed; left: 0; top: 0; transform: translateX(-100%); z-index: 150; box-shadow: var(--shadow-lg); }
  .sidebar.open { transform: translateX(0); }
  .sidebar-overlay.open { display: block; }
  .mobile-toggle { display: flex; }
  .hero { padding: 32px 20px 24px; }
  .hero h1 { font-size: 24px; }
  .chapter-hero { padding: 32px 20px 20px; }
  .chapter-hero h1 { font-size: 22px; }
  .content-area { padding: 0 20px 40px; }
  .k-grid { grid-template-columns: 1fr; }
  .k-block-grid { grid-template-columns: 1fr; }
  .compare-grid { grid-template-columns: 1fr; }
  .footer { padding: 20px 20px; }
  .back-top { right: 80px; bottom: 80px; }
}

/* ========== Utility ========== */
.c-blue { color: var(--accent-blue); }
.c-cyan { color: var(--accent-cyan); }
.c-purple { color: var(--accent-purple); }
.c-green { color: var(--accent-green); }
.c-orange { color: var(--accent-orange); }
.c-red { color: var(--accent-red); }
.c-pink { color: var(--accent-pink); }
.bg-blue { background: rgba(59,130,246,0.1); }
.bg-cyan { background: rgba(6,182,212,0.1); }
.bg-purple { background: rgba(139,92,246,0.1); }
.bg-green { background: rgba(16,185,129,0.1); }
.bg-orange { background: rgba(245,158,11,0.1); }
.bg-red { background: rgba(239,68,68,0.1); }
.bg-pink { background: rgba(236,72,153,0.1); }
.border-blue { border-left-color: var(--accent-blue) !important; }
.border-cyan { border-left-color: var(--accent-cyan) !important; }
.border-purple { border-left-color: var(--accent-purple) !important; }
.border-green { border-left-color: var(--accent-green) !important; }
.border-orange { border-left-color: var(--accent-orange) !important; }
.border-red { border-left-color: var(--accent-red) !important; }
.border-pink { border-left-color: var(--accent-pink) !important; }
.text-muted { color: var(--text-muted); }
.mt-2 { margin-top: 8px; }
.mt-4 { margin-top: 16px; }

/* ========== Diagrams & Charts ========== */
.diagram-box {
  background: var(--bg-card); border: 1px solid var(--border); border-radius: var(--radius-md);
  padding: 20px; margin: 16px 0; overflow-x: auto;
}
.diagram-title {
  font-size: 13px; font-weight: 600; color: var(--text-primary);
  margin-bottom: 14px; text-align: center;
}

/* --- Flowchart (horizontal steps) --- */
.flow-h {
  display: flex; align-items: center; justify-content: center; gap: 0; flex-wrap: wrap;
}
.flow-step {
  background: rgba(59,130,246,0.1); border: 1px solid rgba(59,130,246,0.3);
  border-radius: var(--radius-sm); padding: 10px 16px; font-size: 12px;
  color: var(--text-primary); text-align: center; min-width: 80px;
}
.flow-step.green { background: rgba(16,185,129,0.1); border-color: rgba(16,185,129,0.3); }
.flow-step.orange { background: rgba(245,158,11,0.1); border-color: rgba(245,158,11,0.3); }
.flow-step.red { background: rgba(239,68,68,0.1); border-color: rgba(239,68,68,0.3); }
.flow-step.purple { background: rgba(139,92,246,0.1); border-color: rgba(139,92,246,0.3); }
.flow-arrow {
  width: 24px; height: 2px; background: var(--accent-blue); position: relative; flex-shrink: 0;
}
.flow-arrow::after {
  content: ''; position: absolute; right: 0; top: -3px;
  border: 4px solid transparent; border-left: 6px solid var(--accent-blue);
}
.flow-arrow.dashed { background: repeating-linear-gradient(90deg, var(--accent-blue), var(--accent-blue) 4px, transparent 4px, transparent 8px); }

/* --- Layer cake (vertical stack) --- */
.layer-stack {
  display: flex; flex-direction: column; gap: 2px; max-width: 500px; margin: 0 auto;
}
.layer {
  padding: 10px 16px; border-radius: var(--radius-sm); font-size: 12px;
  text-align: center; color: var(--text-primary); border: 1px solid;
}
.layer.l1 { background: rgba(59,130,246,0.15); border-color: rgba(59,130,246,0.4); }
.layer.l2 { background: rgba(6,182,212,0.12); border-color: rgba(6,182,212,0.35); }
.layer.l3 { background: rgba(139,92,246,0.12); border-color: rgba(139,92,246,0.35); }
.layer.l4 { background: rgba(16,185,129,0.12); border-color: rgba(16,185,129,0.35); }
.layer.l5 { background: rgba(245,158,11,0.12); border-color: rgba(245,158,11,0.35); }
.layer.l6 { background: rgba(239,68,68,0.12); border-color: rgba(239,68,68,0.35); }
.layer.l7 { background: rgba(236,72,153,0.12); border-color: rgba(236,72,153,0.35); }

/* --- State machine --- */
.state-diagram {
  display: flex; align-items: center; justify-content: center; gap: 12px; flex-wrap: wrap; padding: 10px 0;
}
.state-node {
  width: 64px; height: 64px; border-radius: 50%; border: 2px solid var(--accent-blue);
  background: rgba(59,130,246,0.1); display: flex; align-items: center; justify-content: center;
  font-size: 11px; color: var(--text-primary); text-align: center; line-height: 1.3;
}
.state-node.active { border-color: var(--accent-green); background: rgba(16,185,129,0.15); }
.state-node.wait { border-color: var(--accent-orange); background: rgba(245,158,11,0.15); }
.state-conn {
  display: flex; flex-direction: column; align-items: center; gap: 2px;
}
.state-conn .arrow { font-size: 14px; color: var(--accent-blue); }
.state-conn .label { font-size: 10px; color: var(--text-muted); }

/* --- Two-phase commit timeline --- */
.timeline {
  display: flex; flex-direction: column; gap: 16px; max-width: 600px; margin: 0 auto;
}
.timeline-row {
  display: flex; align-items: center; gap: 12px;
}
.timeline-role {
  width: 90px; font-size: 11px; font-weight: 600; color: var(--accent-cyan); text-align: right; flex-shrink: 0;
}
.timeline-bar {
  flex: 1; height: 32px; border-radius: var(--radius-sm); display: flex;
  align-items: center; padding: 0 12px; font-size: 12px; color: var(--text-primary);
  border: 1px solid;
}
.timeline-bar.prepare { background: rgba(59,130,246,0.1); border-color: rgba(59,130,246,0.3); }
.timeline-bar.commit { background: rgba(16,185,129,0.1); border-color: rgba(16,185,129,0.3); }
.timeline-bar.rollback { background: rgba(239,68,68,0.1); border-color: rgba(239,68,68,0.3); }

/* --- Comparison bars --- */
.compare-bars {
  display: flex; flex-direction: column; gap: 10px;
}
.compare-row {
  display: flex; align-items: center; gap: 12px;
}
.compare-label {
  width: 100px; font-size: 12px; color: var(--text-secondary); text-align: right; flex-shrink: 0;
}
.compare-bar-wrap {
  flex: 1; height: 24px; background: var(--bg-card-hover); border-radius: 4px; overflow: hidden;
  border: 1px solid var(--border);
}
.compare-bar-fill {
  height: 100%; border-radius: 4px; display: flex; align-items: center;
  padding-left: 10px; font-size: 11px; color: white; font-weight: 600;
  transition: width 0.5s ease;
}
.compare-bar-fill.blue { background: var(--accent-blue); }
.compare-bar-fill.green { background: var(--accent-green); }
.compare-bar-fill.orange { background: var(--accent-orange); }
.compare-bar-fill.red { background: var(--accent-red); }
.compare-bar-fill.purple { background: var(--accent-purple); }

/* --- Architecture diagram (boxes + arrows) --- */
.arch-grid {
  display: grid; grid-template-columns: repeat(auto-fit, minmax(120px, 1fr)); gap: 12px;
}
.arch-box {
  background: rgba(59,130,246,0.08); border: 1px solid rgba(59,130,246,0.25);
  border-radius: var(--radius-sm); padding: 12px; text-align: center; font-size: 12px;
  color: var(--text-primary); line-height: 1.6;
}
.arch-box.green { background: rgba(16,185,129,0.08); border-color: rgba(16,185,129,0.25); }
.arch-box.orange { background: rgba(245,158,11,0.08); border-color: rgba(245,158,11,0.25); }
.arch-box.red { background: rgba(239,68,68,0.08); border-color: rgba(239,68,68,0.25); }
.arch-box.purple { background: rgba(139,92,246,0.08); border-color: rgba(139,92,246,0.25); }
.arch-arrow-down {
  text-align: center; font-size: 18px; color: var(--accent-blue); line-height: 1;
}

/* --- Venn / Overlap diagram --- */
.venn-diagram {
  display: flex; align-items: center; justify-content: center; gap: 0; padding: 20px 0;
}
.venn-circle {
  width: 100px; height: 100px; border-radius: 50%; border: 2px solid;
  display: flex; align-items: center; justify-content: center; font-size: 11px;
  text-align: center; line-height: 1.4; color: var(--text-primary); margin: 0 -15px;
}
.venn-circle.c1 { border-color: var(--accent-blue); background: rgba(59,130,246,0.1); }
.venn-circle.c2 { border-color: var(--accent-red); background: rgba(239,68,68,0.1); }
.venn-circle.c3 { border-color: var(--accent-green); background: rgba(16,185,129,0.1); }
.venn-overlap {
  position: relative; z-index: 1; font-size: 10px; color: var(--text-muted);
  background: var(--bg-card); padding: 2px 6px; border-radius: 4px;
}

/* --- SVG inline charts --- */
.svg-chart { width: 100%; max-width: 600px; margin: 0 auto; display: block; }