/* --- Modern Design System: Nghệ Thuật Đối Nhân Xử Thế --- */
:root {
  --font-main: 'Plus Jakarta Sans', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Helvetica, Arial, sans-serif;
  --font-serif: 'Merriweather', Georgia, Cambria, 'Times New Roman', Times, serif;

  /* Theme: Dark (Default) */
  --bg-body: #0b0f19;
  --bg-surface: #111827;
  --bg-card: #1f2937;
  --bg-card-hover: #283548;
  --bg-subtle: #1e293b;
  --border-color: rgba(255, 255, 255, 0.08);
  --border-hover: rgba(255, 255, 255, 0.16);
  
  --text-primary: #f9fafb;
  --text-secondary: #9ca3af;
  --text-muted: #6b7280;
  
  --accent-primary: #6366f1;
  --accent-primary-hover: #4f46e5;
  --accent-gradient: linear-gradient(135deg, #6366f1 0%, #8b5cf6 50%, #d946ef 100%);
  --accent-glow: rgba(99, 102, 241, 0.35);

  --color-success: #10b981;
  --color-success-bg: rgba(16, 185, 129, 0.15);
  --color-success-border: rgba(16, 185, 129, 0.4);

  --color-error: #f43f5e;
  --color-error-bg: rgba(244, 63, 94, 0.15);
  --color-error-border: rgba(244, 63, 94, 0.4);

  --color-warning: #f59e0b;
  --color-warning-bg: rgba(245, 158, 11, 0.15);
  
  --radius-sm: 8px;
  --radius-md: 12px;
  --radius-lg: 18px;
  --radius-xl: 24px;
  
  --shadow-sm: 0 2px 4px rgba(0, 0, 0, 0.2);
  --shadow-md: 0 4px 14px rgba(0, 0, 0, 0.3);
  --shadow-lg: 0 10px 30px rgba(0, 0, 0, 0.45);
  --shadow-glow: 0 0 25px var(--accent-glow);

  --reader-font-size: 17px;
  --reader-line-height: 1.8;
  --transition-fast: 0.15s ease;
  --transition-normal: 0.25s ease;
  --transition-slow: 0.4s cubic-bezier(0.16, 1, 0.3, 1);
}

[data-theme="light"] {
  --bg-body: #f8fafc;
  --bg-surface: #ffffff;
  --bg-card: #ffffff;
  --bg-card-hover: #f1f5f9;
  --bg-subtle: #f1f5f9;
  --border-color: rgba(0, 0, 0, 0.08);
  --border-hover: rgba(0, 0, 0, 0.15);
  --text-primary: #0f172a;
  --text-secondary: #475569;
  --text-muted: #94a3b8;
  --shadow-sm: 0 1px 3px rgba(0, 0, 0, 0.05);
  --shadow-md: 0 4px 12px rgba(0, 0, 0, 0.08);
  --shadow-lg: 0 12px 30px rgba(0, 0, 0, 0.12);
  --shadow-glow: 0 0 20px rgba(99, 102, 241, 0.2);
}

[data-theme="sepia"] {
  --bg-body: #f7efe1;
  --bg-surface: #f1e7d2;
  --bg-card: #fdf7ec;
  --bg-card-hover: #eae0ca;
  --bg-subtle: #eae0ca;
  --border-color: rgba(92, 64, 33, 0.12);
  --border-hover: rgba(92, 64, 33, 0.22);
  --text-primary: #3b2a1a;
  --text-secondary: #6e543c;
  --text-muted: #9c836c;
  --accent-primary: #b45309;
  --accent-primary-hover: #92400e;
  --accent-gradient: linear-gradient(135deg, #b45309 0%, #d97706 100%);
  --accent-glow: rgba(180, 83, 9, 0.25);
  --shadow-sm: 0 1px 3px rgba(60, 40, 20, 0.06);
  --shadow-md: 0 4px 12px rgba(60, 40, 20, 0.1);
}

/* Reset & Base */
*, *::before, *::after {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
  -webkit-tap-highlight-color: transparent;
}

html {
  scroll-behavior: smooth;
}

body {
  font-family: var(--font-main);
  background-color: var(--bg-body);
  color: var(--text-primary);
  line-height: 1.6;
  min-height: 100vh;
  min-height: -webkit-fill-available;
  display: flex;
  flex-direction: column;
  overflow-x: hidden;
  touch-action: manipulation;
  transition: background-color var(--transition-normal), color var(--transition-normal);
}

/* Custom Scrollbar */
::-webkit-scrollbar {
  width: 6px;
  height: 6px;
}
::-webkit-scrollbar-track {
  background: var(--bg-body);
}
::-webkit-scrollbar-thumb {
  background: var(--border-color);
  border-radius: 4px;
}
::-webkit-scrollbar-thumb:hover {
  background: var(--text-muted);
}

/* Buttons Base Touch Target */
button {
  touch-action: manipulation;
  user-select: none;
  -webkit-user-select: none;
}

/* ===================================================
   APP HEADER
   =================================================== */
.app-header {
  position: sticky;
  top: 0;
  z-index: 100;
  background: var(--bg-surface);
  border-bottom: 1px solid var(--border-color);
  backdrop-filter: blur(14px);
  -webkit-backdrop-filter: blur(14px);
  box-shadow: var(--shadow-sm);
  padding-top: env(safe-area-inset-top, 0px);
}

.header-inner {
  max-width: 1400px;
  margin: 0 auto;
  padding: 10px 16px;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 12px;
}

.brand-wrapper {
  display: flex;
  align-items: center;
  gap: 10px;
  text-decoration: none;
  cursor: pointer;
  min-width: 0;
  flex-shrink: 1;
}

.brand-logo-img {
  width: 38px;
  height: 38px;
  border-radius: var(--radius-md);
  object-fit: cover;
  box-shadow: var(--shadow-sm);
  border: 1px solid var(--border-color);
  flex-shrink: 0;
}

.brand-info {
  min-width: 0;
}

.brand-info h1 {
  font-size: 1.05rem;
  font-weight: 700;
  letter-spacing: -0.02em;
  background: var(--accent-gradient);
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  line-height: 1.2;
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
}

.brand-subtitle {
  font-size: 0.75rem;
  color: var(--text-secondary);
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
  display: block;
}

/* Desktop Nav Tabs */
.desktop-nav-tabs {
  display: flex;
  align-items: center;
  gap: 6px;
  background: var(--bg-subtle);
  padding: 4px;
  border-radius: var(--radius-lg);
  border: 1px solid var(--border-color);
}

.nav-tab-btn {
  display: flex;
  align-items: center;
  gap: 8px;
  padding: 8px 16px;
  font-family: inherit;
  font-size: 0.88rem;
  font-weight: 600;
  color: var(--text-secondary);
  background: transparent;
  border: none;
  border-radius: var(--radius-md);
  cursor: pointer;
  transition: all var(--transition-fast);
  white-space: nowrap;
}

.nav-tab-btn:hover {
  color: var(--text-primary);
  background: rgba(255, 255, 255, 0.05);
}

.nav-tab-btn.active {
  color: #ffffff;
  background: var(--accent-gradient);
  box-shadow: 0 2px 8px var(--accent-glow);
}

/* Header Controls */
.header-controls {
  display: flex;
  align-items: center;
  gap: 6px;
  flex-shrink: 0;
}

.control-btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: 36px;
  height: 36px;
  border-radius: var(--radius-md);
  background: var(--bg-subtle);
  border: 1px solid var(--border-color);
  color: var(--text-primary);
  cursor: pointer;
  font-weight: 700;
  font-size: 0.82rem;
  transition: all var(--transition-fast);
}

.control-btn:active {
  transform: scale(0.92);
}

.control-btn:hover {
  background: var(--bg-card-hover);
  border-color: var(--border-hover);
}

/* ===================================================
   MAIN CONTENT CONTAINER
   =================================================== */
.main-content {
  flex: 1;
  max-width: 1400px;
  width: 100%;
  margin: 0 auto;
  padding: 20px 16px 40px;
}

.tab-pane {
  display: none;
  animation: fadeIn 0.25s ease;
}

.tab-pane.active {
  display: block;
}

@keyframes fadeIn {
  from { opacity: 0; transform: translateY(6px); }
  to { opacity: 1; transform: translateY(0); }
}

/* ===================================================
   TAB 1: ĐỌC TOÀN VĂN (FULL READER)
   =================================================== */
.reader-layout {
  display: grid;
  grid-template-columns: 320px 1fr;
  gap: 24px;
  align-items: start;
}

/* Desktop Sidebar TOC */
.reader-sidebar {
  background: var(--bg-card);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-lg);
  padding: 16px;
  position: sticky;
  top: 80px;
  max-height: calc(100vh - 100px);
  overflow-y: auto;
  display: flex;
  flex-direction: column;
  gap: 12px;
}

.sidebar-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding-bottom: 8px;
  border-bottom: 1px solid var(--border-color);
}

.sidebar-header h3 {
  font-size: 0.95rem;
  font-weight: 700;
  color: var(--text-primary);
}

.mobile-toc-close-btn {
  display: none;
  background: var(--bg-subtle);
  border: 1px solid var(--border-color);
  color: var(--text-primary);
  width: 32px;
  height: 32px;
  border-radius: 50%;
  font-size: 1rem;
  cursor: pointer;
  align-items: center;
  justify-content: center;
}

.toc-search-input {
  width: 100%;
  padding: 8px 12px;
  border-radius: var(--radius-md);
  background: var(--bg-subtle);
  border: 1px solid var(--border-color);
  color: var(--text-primary);
  font-size: 0.85rem;
  outline: none;
  transition: border-color var(--transition-fast);
}
.toc-search-input:focus {
  border-color: var(--accent-primary);
}

.toc-tree {
  display: flex;
  flex-direction: column;
  gap: 6px;
}

.toc-chapter-item {
  border-radius: var(--radius-sm);
  overflow: hidden;
}

.toc-chapter-title {
  padding: 10px 12px;
  font-size: 0.88rem;
  font-weight: 600;
  color: var(--text-primary);
  background: var(--bg-subtle);
  cursor: pointer;
  display: flex;
  align-items: center;
  justify-content: space-between;
  transition: background var(--transition-fast);
}
.toc-chapter-title:hover {
  background: var(--bg-card-hover);
}

.toc-badge-num {
  font-size: 0.75rem;
  padding: 2px 7px;
  border-radius: 999px;
  background: rgba(255, 255, 255, 0.08);
  color: var(--text-muted);
}

.toc-sections-list {
  padding: 4px 0 4px 6px;
  display: flex;
  flex-direction: column;
  gap: 2px;
}

.toc-sec-btn {
  width: 100%;
  text-align: left;
  padding: 8px 10px;
  border-radius: var(--radius-sm);
  background: transparent;
  border: none;
  color: var(--text-secondary);
  font-size: 0.82rem;
  cursor: pointer;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 8px;
  transition: all var(--transition-fast);
  min-height: 38px;
}

.toc-sec-btn:hover {
  color: var(--text-primary);
  background: rgba(255, 255, 255, 0.04);
}

.toc-sec-btn.active {
  color: #ffffff;
  background: var(--accent-primary);
  font-weight: 600;
}

.toc-read-badge {
  font-size: 0.8rem;
  color: var(--color-success);
  font-weight: 700;
}

/* Mobile Bar Inside Reader */
.reader-mobile-bar {
  display: none;
  align-items: center;
  justify-content: space-between;
  gap: 12px;
  margin-bottom: 16px;
  padding: 8px 12px;
  background: var(--bg-subtle);
  border-radius: var(--radius-md);
  border: 1px solid var(--border-color);
}

.mobile-toc-trigger-btn {
  font-size: 0.85rem;
  padding: 7px 12px;
}

.reader-mobile-progress {
  font-size: 0.82rem;
  font-weight: 700;
  color: var(--accent-primary);
}

/* Reader Main Content Area */
.reader-main-area {
  background: var(--bg-card);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-lg);
  padding: 28px 36px;
  box-shadow: var(--shadow-sm);
}

.reader-toolbar {
  display: flex;
  align-items: center;
  justify-content: space-between;
  flex-wrap: wrap;
  gap: 14px;
  padding-bottom: 18px;
  margin-bottom: 20px;
  border-bottom: 1px solid var(--border-color);
}

.reader-breadcrumbs {
  font-size: 0.85rem;
  color: var(--text-muted);
}
.reader-breadcrumbs span {
  color: var(--accent-primary);
  font-weight: 600;
}

.reader-actions {
  display: flex;
  align-items: center;
  gap: 10px;
  flex-wrap: wrap;
}

.btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: 8px;
  padding: 9px 16px;
  font-family: inherit;
  font-size: 0.88rem;
  font-weight: 600;
  border-radius: var(--radius-md);
  border: none;
  cursor: pointer;
  transition: all var(--transition-fast);
  text-decoration: none;
  min-height: 40px;
}

.btn:active {
  transform: scale(0.97);
}

.btn-primary {
  background: var(--accent-gradient);
  color: #ffffff;
  box-shadow: 0 2px 10px var(--accent-glow);
}
.btn-primary:hover {
  box-shadow: 0 4px 16px var(--accent-glow);
}

.btn-secondary {
  background: var(--bg-subtle);
  color: var(--text-primary);
  border: 1px solid var(--border-color);
}
.btn-secondary:hover {
  background: var(--bg-card-hover);
  border-color: var(--border-hover);
}

.btn-outline-success {
  background: var(--color-success-bg);
  color: var(--color-success);
  border: 1px solid var(--color-success-border);
}

.btn-sm {
  padding: 6px 12px;
  font-size: 0.82rem;
  min-height: 34px;
}

.btn-shuffle {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: 6px;
  transition: all var(--transition-fast);
  cursor: pointer;
  user-select: none;
}

.btn-shuffle .shuffle-icon {
  display: inline-block;
  font-size: 1rem;
  line-height: 1;
  transition: transform 0.35s cubic-bezier(0.34, 1.56, 0.64, 1);
}

.btn-shuffle:hover .shuffle-icon {
  transform: rotate(180deg) scale(1.15);
}

.btn-shuffle.is-shuffling .shuffle-icon {
  animation: shuffle-spin 0.55s cubic-bezier(0.34, 1.56, 0.64, 1);
}

@keyframes shuffle-spin {
  0% { transform: rotate(0deg) scale(1); }
  50% { transform: rotate(180deg) scale(1.25); }
  100% { transform: rotate(360deg) scale(1); }
}

/* Toast Notifications */
.app-toast {
  position: fixed;
  bottom: 84px;
  left: 50%;
  transform: translateX(-50%) translateY(16px);
  background: rgba(30, 41, 59, 0.95);
  color: #f8fafc;
  padding: 10px 20px;
  border-radius: 999px;
  font-size: 0.88rem;
  font-weight: 600;
  box-shadow: 0 10px 25px -5px rgba(0, 0, 0, 0.5), 0 0 0 1px rgba(255, 255, 255, 0.12);
  backdrop-filter: blur(12px);
  -webkit-backdrop-filter: blur(12px);
  z-index: 9999;
  opacity: 0;
  pointer-events: none;
  transition: opacity 0.25s ease, transform 0.25s cubic-bezier(0.34, 1.56, 0.64, 1);
  display: flex;
  align-items: center;
  gap: 8px;
  max-width: 90vw;
  text-align: center;
}

.app-toast.show {
  opacity: 1;
  transform: translateX(-50%) translateY(0);
}

[data-theme="light"] .app-toast {
  background: rgba(255, 255, 255, 0.95);
  color: #0f172a;
  box-shadow: 0 10px 25px -5px rgba(0, 0, 0, 0.15), 0 0 0 1px rgba(0, 0, 0, 0.1);
}

/* Article Typography */
.book-article-header {
  margin-bottom: 24px;
}

.article-chapter-tag {
  display: inline-block;
  font-size: 0.78rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  color: var(--accent-primary);
  margin-bottom: 6px;
}

.article-title {
  font-size: clamp(1.35rem, 3.5vw, 1.85rem);
  font-weight: 800;
  line-height: 1.35;
  margin-bottom: 12px;
  color: var(--text-primary);
}

.article-technique-badge {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  background: rgba(99, 102, 241, 0.12);
  border: 1px solid rgba(99, 102, 241, 0.3);
  color: #818cf8;
  padding: 4px 12px;
  border-radius: 999px;
  font-size: 0.82rem;
  font-weight: 600;
  margin-top: 4px;
}

.book-article-body {
  font-size: var(--reader-font-size);
  line-height: var(--reader-line-height);
  color: var(--text-primary);
}

.book-article-body p {
  margin-bottom: 1.35em;
  word-break: break-word;
}

.book-article-body strong {
  font-weight: 700;
  color: var(--text-primary);
}

.book-article-body em {
  font-style: italic;
}

.book-article-body blockquote {
  margin: 1.5em 0;
  padding: 14px 18px;
  border-left: 4px solid var(--accent-primary);
  background: var(--bg-subtle);
  border-radius: 0 var(--radius-md) var(--radius-md) 0;
  font-style: italic;
  color: var(--text-primary);
  word-break: break-word;
}

.book-article-body ul, .book-article-body ol {
  margin: 1.2em 0 1.5em 1.4em;
}
.book-article-body li {
  margin-bottom: 0.5em;
}

.book-figure {
  margin: 1.8em 0;
  text-align: center;
}

.book-img {
  max-width: 100%;
  height: auto;
  max-height: 480px;
  border-radius: var(--radius-md);
  border: 1px solid var(--border-color);
  box-shadow: var(--shadow-md);
  background: #ffffff;
}

.book-figure figcaption {
  margin-top: 8px;
  font-size: 0.82rem;
  color: var(--text-muted);
}

/* Bottom Reader Nav */
.reader-bottom-nav {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 12px;
  margin-top: 36px;
  padding-top: 20px;
  border-top: 1px solid var(--border-color);
}

/* ===================================================
   TAB 2: QUIZ SYSTEM
   =================================================== */
.quiz-container {
  max-width: 860px;
  margin: 0 auto;
}

.quiz-header-card {
  background: var(--bg-card);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-lg);
  padding: 16px 20px;
  margin-bottom: 20px;
  box-shadow: var(--shadow-sm);
}

.quiz-filters-row {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  justify-content: space-between;
  gap: 12px;
}

.filter-group {
  display: flex;
  align-items: center;
  gap: 8px;
}

.filter-group label {
  font-size: 0.85rem;
  font-weight: 600;
  color: var(--text-secondary);
  white-space: nowrap;
}

.select-custom {
  padding: 8px 12px;
  border-radius: var(--radius-md);
  background: var(--bg-subtle);
  border: 1px solid var(--border-color);
  color: var(--text-primary);
  font-family: inherit;
  font-size: 0.85rem;
  font-weight: 500;
  outline: none;
  cursor: pointer;
  max-width: 100%;
}
.select-custom:focus {
  border-color: var(--accent-primary);
}

.quiz-mode-toggles {
  display: flex;
  gap: 4px;
  background: var(--bg-subtle);
  padding: 3px;
  border-radius: var(--radius-md);
}

.mode-toggle-btn {
  padding: 6px 12px;
  border-radius: var(--radius-sm);
  border: none;
  background: transparent;
  color: var(--text-secondary);
  font-size: 0.82rem;
  font-weight: 600;
  cursor: pointer;
  transition: all var(--transition-fast);
  white-space: nowrap;
}
.mode-toggle-btn.active {
  background: var(--accent-primary);
  color: #ffffff;
}

/* Quiz Card */
.quiz-card {
  background: var(--bg-card);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-xl);
  padding: 24px 28px;
  box-shadow: var(--shadow-md);
  position: relative;
  overflow: hidden;
}

.quiz-meta-bar {
  display: flex;
  align-items: center;
  justify-content: space-between;
  flex-wrap: wrap;
  gap: 10px;
  margin-bottom: 16px;
}

.quiz-badge-wrap {
  display: flex;
  align-items: center;
  gap: 8px;
  flex-wrap: wrap;
}

.badge {
  padding: 4px 10px;
  border-radius: 999px;
  font-size: 0.78rem;
  font-weight: 600;
}
.badge-chapter {
  background: rgba(99, 102, 241, 0.15);
  color: #a5b4fc;
}
.badge-technique {
  background: rgba(245, 158, 11, 0.15);
  color: #fcd34d;
}

.quiz-question-counter {
  font-size: 0.88rem;
  font-weight: 700;
  color: var(--text-secondary);
}

.quiz-question-title {
  font-size: clamp(1.1rem, 2.8vw, 1.3rem);
  font-weight: 700;
  line-height: 1.45;
  color: var(--text-primary);
  margin-bottom: 20px;
}

/* Option List */
.quiz-options-list {
  display: flex;
  flex-direction: column;
  gap: 10px;
  margin-bottom: 20px;
}

.quiz-option-btn {
  width: 100%;
  display: flex;
  align-items: center;
  gap: 14px;
  padding: 14px 18px;
  background: var(--bg-subtle);
  border: 2px solid var(--border-color);
  border-radius: var(--radius-lg);
  color: var(--text-primary);
  font-family: inherit;
  font-size: 0.95rem;
  font-weight: 500;
  text-align: left;
  cursor: pointer;
  transition: all var(--transition-fast);
  position: relative;
  min-height: 54px;
}

.quiz-option-btn:hover:not(:disabled) {
  background: var(--bg-card-hover);
  border-color: var(--accent-primary);
}

.quiz-option-btn:active:not(:disabled) {
  transform: scale(0.98);
}

.option-prefix {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: 32px;
  height: 32px;
  border-radius: 50%;
  background: rgba(255, 255, 255, 0.08);
  font-weight: 700;
  font-size: 0.85rem;
  flex-shrink: 0;
  color: var(--text-primary);
}

.option-text {
  flex: 1;
  word-break: break-word;
}

.quiz-option-btn.correct {
  background: var(--color-success-bg) !important;
  border-color: var(--color-success) !important;
  color: var(--text-primary);
}
.quiz-option-btn.correct .option-prefix {
  background: var(--color-success);
  color: #ffffff;
}

.quiz-option-btn.incorrect {
  background: var(--color-error-bg) !important;
  border-color: var(--color-error) !important;
  color: var(--text-primary);
}
.quiz-option-btn.incorrect .option-prefix {
  background: var(--color-error);
  color: #ffffff;
}

/* Explanation Box */
.quiz-explanation-box {
  margin-top: 20px;
  padding: 18px 20px;
  background: var(--bg-subtle);
  border-radius: var(--radius-lg);
  border: 1px solid var(--border-color);
  animation: fadeIn 0.3s ease;
}

.explanation-header {
  display: flex;
  align-items: center;
  gap: 8px;
  font-size: 1rem;
  font-weight: 700;
  margin-bottom: 10px;
}
.explanation-header.correct { color: var(--color-success); }
.explanation-header.incorrect { color: var(--color-error); }

.explanation-body {
  font-size: 0.92rem;
  line-height: 1.6;
  color: var(--text-primary);
  margin-bottom: 14px;
}

.book-quote-callout {
  padding: 12px 14px;
  background: rgba(245, 158, 11, 0.08);
  border-left: 3px solid var(--color-warning);
  border-radius: 0 var(--radius-sm) var(--radius-sm) 0;
  font-size: 0.88rem;
  font-style: italic;
  color: var(--text-primary);
  margin-bottom: 14px;
}

.explanation-actions {
  display: flex;
  gap: 10px;
  flex-wrap: wrap;
}

/* Quiz Controls Bar */
.quiz-controls-bar {
  display: flex;
  align-items: center;
  justify-content: space-between;
  margin-top: 20px;
  gap: 12px;
}

/* Exam Timer Bar */
.exam-timer-bar {
  display: flex;
  align-items: center;
  justify-content: space-between;
  background: rgba(99, 102, 241, 0.1);
  border: 1px solid rgba(99, 102, 241, 0.3);
  padding: 10px 16px;
  border-radius: var(--radius-md);
  margin-bottom: 16px;
  font-weight: 700;
  color: #a5b4fc;
}

/* Exam Result Card */
.exam-result-card {
  background: var(--bg-card);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-xl);
  padding: 32px 20px;
  text-align: center;
  box-shadow: var(--shadow-lg);
}

.result-score-circle {
  width: 120px;
  height: 120px;
  margin: 0 auto 20px;
  border-radius: 50%;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  background: var(--accent-gradient);
  box-shadow: var(--shadow-glow);
  color: #ffffff;
}

.score-num {
  font-size: 2.2rem;
  font-weight: 800;
  line-height: 1;
}
.score-total {
  font-size: 0.85rem;
  opacity: 0.85;
}

.result-rank-title {
  font-size: 1.4rem;
  font-weight: 800;
  margin-bottom: 8px;
  background: var(--accent-gradient);
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
}

.result-message {
  font-size: 0.95rem;
  color: var(--text-secondary);
  max-width: 500px;
  margin: 0 auto 20px;
}

/* ===================================================
   TAB 3: THẺ GHI NHỚ (FLASHCARDS)
   =================================================== */
.flashcards-container {
  max-width: 720px;
  margin: 0 auto;
}

.fc-gesture-hint {
  font-size: 0.8rem;
  color: var(--text-muted);
}

.flashcard-wrapper {
  perspective: 1000px;
  min-height: 380px;
  margin: 18px 0;
  cursor: pointer;
  touch-action: pan-y;
}

.flashcard-inner {
  position: relative;
  width: 100%;
  min-height: 380px;
  text-align: center;
  transition: transform 0.6s cubic-bezier(0.4, 0, 0.2, 1);
  transform-style: preserve-3d;
}

.flashcard-wrapper.flipped .flashcard-inner {
  transform: rotateY(180deg);
}

.flashcard-front, .flashcard-back {
  position: absolute;
  width: 100%;
  height: 100%;
  min-height: 380px;
  -webkit-backface-visibility: hidden;
  backface-visibility: hidden;
  border-radius: var(--radius-xl);
  border: 1px solid var(--border-color);
  padding: 28px 24px;
  display: flex;
  flex-direction: column;
  justify-content: space-between;
  box-shadow: var(--shadow-md);
  overflow-y: auto;
}

.flashcard-front {
  background: var(--bg-card);
  color: var(--text-primary);
}

.flashcard-back {
  background: var(--bg-surface);
  color: var(--text-primary);
  transform: rotateY(180deg);
  border-color: var(--accent-primary);
}

.fc-badge {
  display: inline-block;
  font-size: 0.78rem;
  font-weight: 700;
  color: var(--accent-primary);
  text-transform: uppercase;
}

.fc-tech-title {
  font-size: clamp(1.2rem, 3vw, 1.5rem);
  font-weight: 800;
  color: var(--text-primary);
  margin: 6px 0;
}

.fc-problem-box {
  margin: auto 0;
  font-size: 1.05rem;
  line-height: 1.6;
  font-weight: 500;
  padding: 10px 0;
}

.fc-solution-box {
  text-align: left;
  margin: auto 0;
  display: flex;
  flex-direction: column;
  gap: 10px;
  padding: 8px 0;
}

.fc-solution-box h4 {
  font-size: 0.92rem;
  font-weight: 700;
  color: var(--color-success);
}

.fc-solution-box p {
  font-size: 0.92rem;
  line-height: 1.55;
}

.fc-hint-text {
  font-size: 0.78rem;
  color: var(--text-muted);
}

.flashcard-actions {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 10px;
  margin-top: 14px;
}

.fc-center-controls {
  display: flex;
  align-items: center;
  gap: 10px;
}

.fc-counter-text {
  font-weight: 700;
  font-size: 0.88rem;
  color: var(--text-secondary);
}

/* ===================================================
   TAB 4: DASHBOARD
   =================================================== */
.dashboard-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(220px, 1fr));
  gap: 16px;
  margin-bottom: 24px;
}

.stat-card {
  background: var(--bg-card);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-lg);
  padding: 18px 20px;
  box-shadow: var(--shadow-sm);
  display: flex;
  align-items: center;
  gap: 14px;
}

.stat-icon {
  width: 48px;
  height: 48px;
  border-radius: var(--radius-md);
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 1.5rem;
  background: var(--bg-subtle);
  flex-shrink: 0;
}

.stat-info h4 {
  font-size: 0.82rem;
  color: var(--text-secondary);
  margin-bottom: 2px;
}

.stat-info .stat-value {
  font-size: 1.5rem;
  font-weight: 800;
  color: var(--text-primary);
  line-height: 1.1;
}

.chapters-progress-card, .badges-showcase-card {
  background: var(--bg-card);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-lg);
  padding: 22px 20px;
  margin-bottom: 24px;
}

.chapter-progress-row {
  display: grid;
  grid-template-columns: 2fr 3fr 60px;
  align-items: center;
  gap: 12px;
  padding: 10px 0;
  border-bottom: 1px solid var(--border-color);
}
.chapter-progress-row:last-child {
  border-bottom: none;
}

.progress-bar-bg {
  width: 100%;
  height: 8px;
  border-radius: 999px;
  background: var(--bg-subtle);
  overflow: hidden;
}

.progress-bar-fill {
  height: 100%;
  border-radius: 999px;
  background: var(--accent-gradient);
  transition: width 0.4s ease;
}

.badges-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(180px, 1fr));
  gap: 14px;
  margin-top: 16px;
}

.badge-item {
  background: var(--bg-subtle);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-md);
  padding: 16px 14px;
  text-align: center;
  transition: all var(--transition-fast);
}

.badge-item.unlocked {
  border-color: rgba(99, 102, 241, 0.4);
  background: rgba(99, 102, 241, 0.08);
}

.badge-item.locked {
  opacity: 0.45;
  filter: grayscale(1);
}

.badge-icon {
  font-size: 2rem;
  margin-bottom: 6px;
}

.badge-name {
  font-size: 0.9rem;
  font-weight: 700;
  margin-bottom: 4px;
}

.badge-desc {
  font-size: 0.78rem;
  color: var(--text-muted);
}

/* ===================================================
   TAB 5: UNIVERSAL SEARCH
   =================================================== */
.search-container {
  max-width: 900px;
  margin: 0 auto;
}

.search-box-large {
  position: relative;
  margin-bottom: 20px;
}

.search-input-main {
  width: 100%;
  padding: 16px 20px 16px 48px;
  border-radius: var(--radius-xl);
  background: var(--bg-card);
  border: 2px solid var(--border-color);
  color: var(--text-primary);
  font-family: inherit;
  font-size: 1rem;
  outline: none;
  box-shadow: var(--shadow-sm);
  transition: all var(--transition-fast);
}

.search-input-main:focus {
  border-color: var(--accent-primary);
  box-shadow: var(--shadow-glow);
}

.search-icon-pos {
  position: absolute;
  left: 18px;
  top: 50%;
  transform: translateY(-50%);
  color: var(--text-muted);
  font-size: 1.2rem;
}

.search-results-list {
  display: flex;
  flex-direction: column;
  gap: 12px;
}

.search-result-item {
  background: var(--bg-card);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-lg);
  padding: 16px 18px;
  cursor: pointer;
  transition: all var(--transition-fast);
}

.search-result-item:hover {
  background: var(--bg-card-hover);
  border-color: var(--accent-primary);
}

.search-result-item:active {
  transform: scale(0.99);
}

.search-result-title {
  font-size: 1rem;
  font-weight: 700;
  color: var(--text-primary);
  margin-bottom: 4px;
}

.search-result-snippet {
  font-size: 0.88rem;
  color: var(--text-secondary);
  line-height: 1.5;
}

.search-result-snippet mark {
  background: rgba(245, 158, 11, 0.35);
  color: var(--text-primary);
  padding: 1px 4px;
  border-radius: 3px;
}

/* ===================================================
   FLOATING BUTTONS
   =================================================== */
.floating-buttons-group {
  position: fixed;
  bottom: calc(74px + env(safe-area-inset-bottom, 12px));
  right: 14px;
  display: flex;
  flex-direction: column;
  gap: 8px;
  z-index: 99;
  pointer-events: none;
}

.floating-action-btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  border-radius: 50%;
  background: var(--accent-gradient);
  color: #ffffff;
  border: 1px solid rgba(255, 255, 255, 0.2);
  font-family: inherit;
  font-size: 1.1rem;
  font-weight: 700;
  cursor: pointer;
  box-shadow: 0 4px 16px var(--accent-glow);
  transition: all var(--transition-fast);
  pointer-events: auto;
  width: 40px;
  height: 40px;
}

.floating-action-btn:active {
  transform: scale(0.92);
}

#floating-scroll-top-btn {
  display: none;
}

/* ===================================================
   MOBILE BOTTOM NAVIGATION BAR
   =================================================== */
.mobile-bottom-nav {
  display: none;
  position: fixed;
  bottom: 0;
  left: 0;
  right: 0;
  width: 100%;
  max-width: 100vw;
  box-sizing: border-box;
  z-index: 1000;
  background: var(--bg-surface);
  border-top: 1px solid var(--border-color);
  backdrop-filter: blur(16px);
  -webkit-backdrop-filter: blur(16px);
  padding: 4px 6px calc(6px + env(safe-area-inset-bottom, 0px));
  box-shadow: 0 -4px 16px rgba(0, 0, 0, 0.25);
  grid-template-columns: repeat(5, 1fr);
  align-items: center;
  height: calc(62px + env(safe-area-inset-bottom, 0px));
}

.mobile-nav-btn {
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  gap: 2px;
  background: transparent;
  border: none;
  color: var(--text-muted);
  font-family: inherit;
  font-size: 0.68rem;
  font-weight: 600;
  cursor: pointer;
  padding: 4px 1px;
  height: 100%;
  width: 100%;
  transition: all var(--transition-fast);
  overflow: hidden;
}

.mobile-nav-icon {
  font-size: 1.2rem;
  line-height: 1;
  transition: transform var(--transition-fast);
}

.mobile-nav-label {
  letter-spacing: -0.02em;
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
  max-width: 100%;
}

.mobile-nav-btn.active {
  color: var(--accent-primary);
}

.mobile-nav-btn.active .mobile-nav-icon {
  transform: scale(1.15);
}

.mobile-nav-btn:active {
  transform: scale(0.94);
}

/* TOC Drawer Overlay */
.toc-drawer-overlay {
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  background: rgba(0, 0, 0, 0.65);
  backdrop-filter: blur(4px);
  -webkit-backdrop-filter: blur(4px);
  z-index: 1999;
  display: none;
  animation: fadeIn 0.2s ease;
}
.toc-drawer-overlay.active {
  display: block;
}

/* Confetti Canvas */
#confetti-canvas {
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  pointer-events: none;
  z-index: 10000;
}

/* ===================================================
   RESPONSIVE DESIGN (TABLET & MOBILE)
   =================================================== */

/* Tablet (max-width: 992px) */
@media (max-width: 992px) {
  .reader-layout {
    grid-template-columns: 280px 1fr;
    gap: 16px;
  }
  .desktop-nav-tabs .nav-tab-btn span {
    font-size: 0.82rem;
  }
}

/* Mobile Devices (max-width: 768px) */
@media (max-width: 768px) {
  /* Hide Desktop Nav, Show Mobile Bottom Nav */
  .desktop-nav-tabs {
    display: none !important;
  }
  .mobile-bottom-nav {
    display: grid !important;
  }
  
  /* Safe padding for bottom bar */
  body {
    padding-bottom: calc(66px + env(safe-area-inset-bottom, 12px));
  }
  .main-content {
    padding: 14px 12px 32px;
  }

  /* Compact Header */
  .header-inner {
    padding: 8px 12px;
  }
  .brand-logo-img {
    width: 32px;
    height: 32px;
  }
  .brand-info h1 {
    font-size: 0.95rem;
  }
  .brand-subtitle {
    display: none;
  }
  .control-btn {
    width: 32px;
    height: 32px;
    font-size: 0.78rem;
  }

  /* Reader Layout & Mobile Drawer TOC */
  .reader-layout {
    grid-template-columns: 1fr;
  }

  .reader-sidebar {
    position: fixed;
    top: 0;
    left: 0;
    bottom: 0;
    width: 86%;
    max-width: 340px;
    height: 100vh;
    height: -webkit-fill-available;
    z-index: 2000;
    transform: translateX(-100%);
    transition: transform 0.3s cubic-bezier(0.16, 1, 0.3, 1);
    box-shadow: var(--shadow-lg);
    border-radius: 0 var(--radius-xl) var(--radius-xl) 0;
    max-height: none;
    padding: 16px 14px calc(20px + env(safe-area-inset-bottom, 0px));
  }

  .reader-sidebar.drawer-open {
    transform: translateX(0);
  }

  .mobile-toc-close-btn {
    display: flex;
  }

  .reader-mobile-bar {
    display: flex;
  }

  .reader-main-area {
    padding: 18px 14px;
    border-radius: var(--radius-md);
  }

  .reader-toolbar {
    flex-direction: column;
    align-items: flex-start;
    gap: 12px;
  }

  .reader-actions {
    width: 100%;
    display: flex;
    flex-direction: column;
    gap: 8px;
  }

  .reader-actions .btn {
    width: 100%;
    justify-content: center;
  }

  .article-title {
    font-size: 1.35rem;
  }

  .reader-bottom-nav {
    flex-direction: row;
    gap: 10px;
  }
  .reader-bottom-nav .btn {
    flex: 1;
    font-size: 0.85rem;
    padding: 10px 8px;
  }

  /* Quiz on Mobile */
  .quiz-card {
    padding: 18px 14px;
    border-radius: var(--radius-lg);
  }

  .quiz-filters-row {
    flex-direction: column;
    align-items: stretch;
    gap: 10px;
  }

  .filter-group {
    width: 100%;
    justify-content: space-between;
  }
  .filter-group .select-custom {
    flex: 1;
    max-width: 75%;
  }

  .quiz-mode-toggles {
    width: 100%;
  }
  .quiz-mode-toggles .mode-toggle-btn {
    flex: 1;
    text-align: center;
  }

  .quiz-option-btn {
    padding: 12px 14px;
    font-size: 0.92rem;
  }

  .quiz-controls-bar .btn {
    flex: 1;
    padding: 10px 8px;
    font-size: 0.85rem;
  }

  /* Flashcards on Mobile */
  .flashcard-wrapper, .flashcard-inner, .flashcard-front, .flashcard-back {
    min-height: 420px;
  }
  .flashcard-front, .flashcard-back {
    padding: 22px 16px;
  }
  .fc-problem-box {
    font-size: 1rem;
  }
  .fc-solution-box h4 {
    font-size: 0.88rem;
  }
  .fc-solution-box p {
    font-size: 0.88rem;
  }
  .flashcard-actions {
    flex-wrap: wrap;
    gap: 8px;
  }
  .flashcard-actions #fc-prev-btn, .flashcard-actions #fc-next-btn {
    flex: 1;
  }
  .fc-center-controls {
    width: 100%;
    order: -1;
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: 6px;
    margin-bottom: 6px;
  }
  .fc-center-controls .btn {
    padding: 7px 10px;
    font-size: 0.8rem;
    min-height: 36px;
  }
  .app-toast {
    bottom: calc(76px + env(safe-area-inset-bottom, 0px));
    font-size: 0.82rem;
    padding: 8px 16px;
  }

  /* Dashboard on Mobile */
  .dashboard-grid {
    grid-template-columns: 1fr 1fr;
    gap: 10px;
  }
  .stat-card {
    padding: 12px;
    gap: 10px;
  }
  .stat-icon {
    width: 38px;
    height: 38px;
    font-size: 1.25rem;
  }
  .stat-info .stat-value {
    font-size: 1.25rem;
  }
  .chapter-progress-row {
    grid-template-columns: 1fr;
    gap: 6px;
  }
  .badges-grid {
    grid-template-columns: 1fr 1fr;
    gap: 10px;
  }

}

/* Extra Small Phones (max-width: 400px) */
@media (max-width: 400px) {
  .dashboard-grid {
    grid-template-columns: 1fr;
  }
  .badges-grid {
    grid-template-columns: 1fr;
  }
  .header-controls {
    gap: 3px;
  }
  .control-btn {
    width: 29px;
    height: 29px;
    font-size: 0.72rem;
  }
}