/* ==========================================================================
   LearnHub - 個人上課教材網站 核心樣式表
   支援淺色 (Light) 與深色 (Dark) 主題切換、響應式卡片目錄與內嵌全螢幕閱讀器
   ========================================================================== */

:root {
  /* 淺色主題變數 */
  --bg-primary: #f8fafc;
  --bg-secondary: #ffffff;
  --bg-tertiary: #f1f5f9;
  --bg-header: rgba(255, 255, 255, 0.88);
  --text-primary: #0f172a;
  --text-secondary: #475569;
  --text-muted: #64748b;
  --border-color: #e2e8f0;
  --border-hover: #cbd5e1;

  /* 品牌強調色 */
  --accent-primary: #2563eb;
  --accent-hover: #1d4ed8;
  --accent-soft: rgba(37, 99, 235, 0.1);
  --accent-border: rgba(37, 99, 235, 0.28);

  /* 難度標籤色彩 */
  --level-beginner-bg: rgba(16, 185, 129, 0.12);
  --level-beginner-text: #059669;
  --level-advanced-bg: rgba(245, 158, 11, 0.14);
  --level-advanced-text: #d97706;
  --level-practical-bg: rgba(139, 92, 246, 0.12);
  --level-practical-text: #7c3aed;

  /* 陰影與圓角 */
  --shadow-sm: 0 1px 2px 0 rgba(15, 23, 42, 0.04);
  --shadow-md: 0 4px 12px -2px rgba(15, 23, 42, 0.07), 0 2px 6px -1px rgba(15, 23, 42, 0.04);
  --shadow-lg: 0 12px 24px -4px rgba(15, 23, 42, 0.1), 0 4px 8px -2px rgba(15, 23, 42, 0.05);
  --radius-sm: 8px;
  --radius-md: 12px;
  --radius-lg: 16px;
  --radius-full: 9999px;

  --header-height: 68px;
  --sidebar-width: 270px;
  --transition: 0.2s cubic-bezier(0.4, 0, 0.2, 1);
}

[data-theme="dark"] {
  --bg-primary: #0b1120;
  --bg-secondary: #151f32;
  --bg-tertiary: #1e293b;
  --bg-header: rgba(15, 23, 42, 0.88);
  --text-primary: #f1f5f9;
  --text-secondary: #cbd5e1;
  --text-muted: #94a3b8;
  --border-color: #1e293b;
  --border-hover: #334155;

  --accent-primary: #3b82f6;
  --accent-hover: #60a5fa;
  --accent-soft: rgba(59, 130, 246, 0.16);
  --accent-border: rgba(59, 130, 246, 0.35);

  --level-beginner-bg: rgba(16, 185, 129, 0.18);
  --level-beginner-text: #34d399;
  --level-advanced-bg: rgba(245, 158, 11, 0.18);
  --level-advanced-text: #fbbf24;
  --level-practical-bg: rgba(139, 92, 246, 0.2);
  --level-practical-text: #a78bfa;

  --shadow-sm: 0 1px 2px 0 rgba(0, 0, 0, 0.25);
  --shadow-md: 0 6px 16px -2px rgba(0, 0, 0, 0.35);
  --shadow-lg: 0 16px 32px -4px rgba(0, 0, 0, 0.5);
}

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

html {
  font-family: "Inter", "Noto Sans TC", "PingFang TC", "Microsoft JhengHei", -apple-system, BlinkMacSystemFont, sans-serif;
  color: var(--text-primary);
  background-color: var(--bg-primary);
  line-height: 1.6;
  -webkit-font-smoothing: antialiased;
}

body {
  min-height: 100vh;
  display: flex;
  flex-direction: column;
  background-color: var(--bg-primary);
  color: var(--text-primary);
  transition: background-color var(--transition), color var(--transition);
}

body.viewer-open {
  overflow: hidden;
}

a {
  color: inherit;
  text-decoration: none;
}

button {
  font-family: inherit;
  cursor: pointer;
  border: none;
  background: none;
  color: inherit;
}

/* ==========================================================================
   頂部導覽列 (Header)
   ========================================================================== */
.site-header {
  position: sticky;
  top: 0;
  z-index: 40;
  height: var(--header-height);
  background: var(--bg-header);
  backdrop-filter: blur(12px);
  -webkit-backdrop-filter: blur(12px);
  border-bottom: 1px solid var(--border-color);
  transition: background-color var(--transition), border-color var(--transition);
}

.header-inner {
  max-width: 1440px;
  height: 100%;
  margin: 0 auto;
  padding: 0 24px;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 20px;
}

.brand {
  display: flex;
  align-items: center;
  gap: 12px;
  cursor: pointer;
  user-select: none;
  flex-shrink: 0;
}

.brand-logo {
  width: 40px;
  height: 40px;
  border-radius: 10px;
  background: linear-gradient(135deg, #2563eb 0%, #4f46e5 100%);
  color: #ffffff;
  display: flex;
  align-items: center;
  justify-content: center;
  box-shadow: 0 4px 10px rgba(37, 99, 235, 0.25);
}

.brand-text {
  display: flex;
  flex-direction: column;
}

.brand-title {
  font-size: 1.2rem;
  font-weight: 700;
  letter-spacing: -0.02em;
  line-height: 1.2;
}

.brand-subtitle {
  font-size: 0.75rem;
  color: var(--text-muted);
}

/* 搜尋欄 */
.search-container {
  flex: 1;
  max-width: 520px;
  position: relative;
}

.search-icon {
  position: absolute;
  left: 14px;
  top: 50%;
  transform: translateY(-50%);
  color: var(--text-muted);
  pointer-events: none;
  display: flex;
}

.search-input {
  width: 100%;
  height: 42px;
  padding: 0 76px 0 42px;
  border-radius: var(--radius-full);
  border: 1px solid var(--border-color);
  background-color: var(--bg-tertiary);
  color: var(--text-primary);
  font-size: 0.92rem;
  transition: all var(--transition);
}

.search-input::placeholder {
  color: var(--text-muted);
}

.search-input:focus {
  outline: none;
  background-color: var(--bg-secondary);
  border-color: var(--accent-primary);
  box-shadow: 0 0 0 3px var(--accent-soft);
}

.search-shortcut {
  position: absolute;
  right: 12px;
  top: 50%;
  transform: translateY(-50%);
  font-size: 0.72rem;
  padding: 2px 7px;
  border-radius: 5px;
  background-color: var(--bg-secondary);
  border: 1px solid var(--border-color);
  color: var(--text-muted);
  pointer-events: none;
}

.search-clear-btn {
  position: absolute;
  right: 12px;
  top: 50%;
  transform: translateY(-50%);
  width: 24px;
  height: 24px;
  border-radius: 50%;
  display: none;
  align-items: center;
  justify-content: center;
  color: var(--text-muted);
  background-color: var(--bg-secondary);
}

.search-clear-btn:hover {
  color: var(--text-primary);
}

/* 右側工具按鈕 */
.header-actions {
  display: flex;
  align-items: center;
  gap: 10px;
  flex-shrink: 0;
}

.icon-btn {
  height: 40px;
  padding: 0 12px;
  border-radius: var(--radius-sm);
  border: 1px solid var(--border-color);
  background-color: var(--bg-secondary);
  color: var(--text-secondary);
  display: inline-flex;
  align-items: center;
  gap: 7px;
  font-size: 0.875rem;
  font-weight: 500;
  transition: all var(--transition);
}

.icon-btn:hover {
  border-color: var(--border-hover);
  color: var(--text-primary);
  background-color: var(--bg-tertiary);
}

.theme-toggle-btn {
  width: 40px;
  padding: 0;
  justify-content: center;
}

/* ==========================================================================
   主要版面架構 (左側分類欄 + 右側主內容區)
   ========================================================================== */
.main-layout {
  flex: 1;
  max-width: 1440px;
  width: 100%;
  margin: 0 auto;
  padding: 28px 24px 48px;
  display: grid;
  grid-template-columns: var(--sidebar-width) 1fr;
  gap: 32px;
  align-items: start;
}

/* 左側分類導覽欄 */
.sidebar {
  position: sticky;
  top: calc(var(--header-height) + 28px);
  display: flex;
  flex-direction: column;
  gap: 24px;
}

.sidebar-section {
  background-color: var(--bg-secondary);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-md);
  padding: 16px;
  box-shadow: var(--shadow-sm);
}

.sidebar-heading {
  font-size: 0.78rem;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--text-muted);
  padding: 0 8px 10px;
  display: flex;
  align-items: center;
  justify-content: space-between;
}

.category-nav {
  display: flex;
  flex-direction: column;
  gap: 4px;
}

.category-btn {
  width: 100%;
  padding: 10px 12px;
  border-radius: var(--radius-sm);
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 10px;
  font-size: 0.92rem;
  font-weight: 500;
  color: var(--text-secondary);
  transition: all var(--transition);
  text-align: left;
}

.category-btn:hover {
  background-color: var(--bg-tertiary);
  color: var(--text-primary);
}

.category-btn.active {
  background-color: var(--accent-soft);
  color: var(--accent-primary);
  font-weight: 600;
}

.category-btn-left {
  display: flex;
  align-items: center;
  gap: 10px;
  min-width: 0;
}

.category-dot {
  width: 10px;
  height: 10px;
  border-radius: 50%;
  flex-shrink: 0;
}

.category-name {
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
}

.category-count {
  font-size: 0.75rem;
  font-weight: 600;
  padding: 2px 8px;
  border-radius: var(--radius-full);
  background-color: var(--bg-tertiary);
  color: var(--text-muted);
  transition: all var(--transition);
}

.category-btn.active .category-count {
  background-color: var(--accent-primary);
  color: #ffffff;
}

/* 標籤快速篩選區 */
.tag-cloud {
  display: flex;
  flex-wrap: wrap;
  gap: 6px;
  padding: 4px 4px 0;
}

.tag-chip {
  font-size: 0.78rem;
  padding: 4px 10px;
  border-radius: var(--radius-full);
  background-color: var(--bg-tertiary);
  color: var(--text-secondary);
  border: 1px solid transparent;
  transition: all var(--transition);
}

.tag-chip:hover {
  border-color: var(--border-hover);
  color: var(--text-primary);
}

.tag-chip.active {
  background-color: var(--accent-soft);
  color: var(--accent-primary);
  border-color: var(--accent-border);
  font-weight: 600;
}

/* 新增教材提示卡 */
.guide-card {
  background: linear-gradient(135deg, var(--accent-soft), transparent);
  border: 1px solid var(--accent-border);
  border-radius: var(--radius-md);
  padding: 16px;
  font-size: 0.84rem;
  color: var(--text-secondary);
}

.guide-card-title {
  font-weight: 600;
  color: var(--text-primary);
  margin-bottom: 6px;
  display: flex;
  align-items: center;
  gap: 6px;
}

.guide-card code {
  font-family: ui-monospace, SFMono-Regular, Menlo, Monaco, Consolas, monospace;
  font-size: 0.78rem;
  padding: 1px 5px;
  border-radius: 4px;
  background-color: var(--bg-secondary);
  border: 1px solid var(--border-color);
  color: var(--accent-primary);
}

/* ==========================================================================
   右側主內容區 (Content Area)
   ========================================================================== */
.content-area {
  display: flex;
  flex-direction: column;
  gap: 24px;
  min-width: 0;
}

/* 分類橫幅介紹 (Banner) */
.category-banner {
  background-color: var(--bg-secondary);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-lg);
  padding: 24px 28px;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 24px;
  box-shadow: var(--shadow-sm);
}

.banner-info {
  display: flex;
  flex-direction: column;
  gap: 6px;
}

.banner-badge {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  font-size: 0.78rem;
  font-weight: 600;
  color: var(--accent-primary);
  background-color: var(--accent-soft);
  padding: 3px 10px;
  border-radius: var(--radius-full);
  width: fit-content;
}

.banner-title {
  font-size: 1.5rem;
  font-weight: 700;
  letter-spacing: -0.02em;
}

.banner-desc {
  color: var(--text-secondary);
  font-size: 0.94rem;
}

.banner-stats {
  display: flex;
  align-items: center;
  gap: 20px;
  flex-shrink: 0;
}

.stat-box {
  text-align: center;
  padding: 10px 18px;
  background-color: var(--bg-tertiary);
  border-radius: var(--radius-md);
}

.stat-value {
  font-size: 1.4rem;
  font-weight: 700;
  color: var(--accent-primary);
  line-height: 1.2;
}

.stat-label {
  font-size: 0.75rem;
  color: var(--text-muted);
}

/* 篩選與排序控制列 */
.toolbar {
  display: flex;
  align-items: center;
  justify-content: space-between;
  flex-wrap: wrap;
  gap: 12px;
}

.toolbar-left {
  display: flex;
  align-items: center;
  gap: 10px;
  flex-wrap: wrap;
}

.result-summary {
  font-size: 0.9rem;
  color: var(--text-secondary);
}

.result-summary strong {
  color: var(--text-primary);
}

.active-filter-pill {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  font-size: 0.8rem;
  padding: 3px 10px;
  border-radius: var(--radius-full);
  background-color: var(--accent-soft);
  color: var(--accent-primary);
  font-weight: 500;
}

.toolbar-right {
  display: flex;
  align-items: center;
  gap: 10px;
}

.sort-select {
  height: 36px;
  padding: 0 12px;
  border-radius: var(--radius-sm);
  border: 1px solid var(--border-color);
  background-color: var(--bg-secondary);
  color: var(--text-secondary);
  font-size: 0.85rem;
  font-family: inherit;
  cursor: pointer;
}

.sort-select:focus {
  outline: none;
  border-color: var(--accent-primary);
}

.view-mode-group {
  display: inline-flex;
  background-color: var(--bg-secondary);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-sm);
  padding: 2px;
}

.view-mode-btn {
  width: 32px;
  height: 30px;
  border-radius: 6px;
  display: flex;
  align-items: center;
  justify-content: center;
  color: var(--text-muted);
  transition: all var(--transition);
}

.view-mode-btn.active {
  background-color: var(--accent-soft);
  color: var(--accent-primary);
}

/* ==========================================================================
   教材卡片網格與列表 (Materials Grid / List)
   ========================================================================== */
.materials-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(320px, 1fr));
  gap: 20px;
}

.materials-grid.list-view {
  grid-template-columns: 1fr;
  gap: 14px;
}

.material-card {
  background-color: var(--bg-secondary);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-md);
  padding: 22px;
  display: flex;
  flex-direction: column;
  justify-content: space-between;
  gap: 16px;
  box-shadow: var(--shadow-sm);
  transition: transform var(--transition), box-shadow var(--transition), border-color var(--transition);
  cursor: pointer;
  position: relative;
  overflow: hidden;
}

.material-card::before {
  content: "";
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 4px;
  background-color: var(--card-accent, var(--accent-primary));
  opacity: 0.85;
}

.material-card:hover {
  transform: translateY(-3px);
  box-shadow: var(--shadow-md);
  border-color: var(--border-hover);
}

.materials-grid.list-view .material-card {
  flex-direction: row;
  align-items: center;
  padding: 18px 24px;
}

.materials-grid.list-view .material-card::before {
  width: 4px;
  height: 100%;
}

.card-top {
  display: flex;
  flex-direction: column;
  gap: 10px;
  flex: 1;
}

.card-meta-row {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 8px;
  flex-wrap: wrap;
}

.card-category-badge {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  font-size: 0.76rem;
  font-weight: 600;
  padding: 3px 10px;
  border-radius: var(--radius-full);
  background-color: var(--bg-tertiary);
  color: var(--text-secondary);
}

.card-level-badge {
  font-size: 0.74rem;
  font-weight: 600;
  padding: 2px 9px;
  border-radius: var(--radius-full);
}

.level-入門 {
  background-color: var(--level-beginner-bg);
  color: var(--level-beginner-text);
}

.level-進階 {
  background-color: var(--level-advanced-bg);
  color: var(--level-advanced-text);
}

.level-實戰 {
  background-color: var(--level-practical-bg);
  color: var(--level-practical-text);
}

.card-title {
  font-size: 1.12rem;
  font-weight: 700;
  line-height: 1.4;
  color: var(--text-primary);
  transition: color var(--transition);
}

.material-card:hover .card-title {
  color: var(--accent-primary);
}

.card-desc {
  font-size: 0.88rem;
  color: var(--text-secondary);
  display: -webkit-box;
  -webkit-line-clamp: 3;
  -webkit-box-orient: vertical;
  overflow: hidden;
}

.card-tags {
  display: flex;
  flex-wrap: wrap;
  gap: 6px;
  margin-top: 2px;
}

.card-tag {
  font-size: 0.74rem;
  padding: 2px 8px;
  border-radius: 5px;
  background-color: var(--bg-tertiary);
  color: var(--text-muted);
}

.card-footer {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding-top: 14px;
  border-top: 1px solid var(--border-color);
  gap: 12px;
}

.materials-grid.list-view .card-footer {
  border-top: none;
  padding-top: 0;
  flex-shrink: 0;
  flex-direction: column;
  align-items: flex-end;
}

.card-info-items {
  display: flex;
  align-items: center;
  gap: 12px;
  font-size: 0.78rem;
  color: var(--text-muted);
}

.card-info-item {
  display: inline-flex;
  align-items: center;
  gap: 4px;
}

.card-actions {
  display: flex;
  align-items: center;
  gap: 6px;
}

.btn-read {
  padding: 6px 12px;
  border-radius: var(--radius-sm);
  background-color: var(--accent-soft);
  color: var(--accent-primary);
  font-size: 0.82rem;
  font-weight: 600;
  display: inline-flex;
  align-items: center;
  gap: 5px;
  transition: all var(--transition);
}

.material-card:hover .btn-read {
  background-color: var(--accent-primary);
  color: #ffffff;
}

.btn-new-tab,
.btn-download {
  width: 32px;
  height: 32px;
  border-radius: var(--radius-sm);
  border: 1px solid var(--border-color);
  display: inline-flex;
  align-items: center;
  justify-content: center;
  color: var(--text-muted);
  transition: all var(--transition);
}

.btn-new-tab:hover,
.btn-download:hover {
  border-color: var(--accent-primary);
  color: var(--accent-primary);
  background-color: var(--accent-soft);
}

/* 空狀態顯示 */
.empty-state {
  background-color: var(--bg-secondary);
  border: 1px dashed var(--border-color);
  border-radius: var(--radius-lg);
  padding: 56px 24px;
  text-align: center;
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 12px;
}

.empty-icon {
  width: 56px;
  height: 56px;
  border-radius: 50%;
  background-color: var(--bg-tertiary);
  color: var(--text-muted);
  display: flex;
  align-items: center;
  justify-content: center;
}

.empty-title {
  font-size: 1.1rem;
  font-weight: 600;
}

.empty-desc {
  font-size: 0.9rem;
  color: var(--text-muted);
  max-width: 400px;
}

.btn-primary {
  margin-top: 6px;
  padding: 8px 18px;
  border-radius: var(--radius-sm);
  background-color: var(--accent-primary);
  color: #ffffff;
  font-size: 0.88rem;
  font-weight: 500;
  transition: background-color var(--transition);
}

.btn-primary:hover {
  background-color: var(--accent-hover);
}

/* ==========================================================================
   站內內嵌 HTML 教材閱讀器 (Modal / Fullscreen Reader)
   ========================================================================== */
.viewer-overlay {
  position: fixed;
  inset: 0;
  z-index: 100;
  background-color: rgba(15, 23, 42, 0.65);
  backdrop-filter: blur(4px);
  display: none;
  align-items: center;
  justify-content: center;
  padding: 20px;
  opacity: 0;
  transition: opacity var(--transition);
}

.viewer-overlay.active {
  display: flex;
  opacity: 1;
}

.viewer-overlay.fullscreen {
  padding: 0;
}

.viewer-container {
  width: 100%;
  max-width: 1320px;
  height: 92vh;
  background-color: var(--bg-secondary);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-lg);
  box-shadow: var(--shadow-lg);
  display: flex;
  flex-direction: column;
  overflow: hidden;
  transition: all var(--transition);
}

.viewer-overlay.fullscreen .viewer-container {
  max-width: 100%;
  height: 100vh;
  border-radius: 0;
  border: none;
}

.viewer-header {
  height: 58px;
  padding: 0 18px;
  background-color: var(--bg-secondary);
  border-bottom: 1px solid var(--border-color);
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 16px;
  flex-shrink: 0;
}

.viewer-header-left {
  display: flex;
  align-items: center;
  gap: 12px;
  min-width: 0;
}

.viewer-back-btn {
  padding: 6px 12px;
  border-radius: var(--radius-sm);
  background-color: var(--bg-tertiary);
  color: var(--text-primary);
  font-size: 0.85rem;
  font-weight: 500;
  display: inline-flex;
  align-items: center;
  gap: 6px;
  flex-shrink: 0;
  transition: background-color var(--transition);
}

.viewer-back-btn:hover {
  background-color: var(--border-color);
}

.viewer-title-group {
  display: flex;
  align-items: center;
  gap: 10px;
  min-width: 0;
}

.viewer-category-tag {
  font-size: 0.75rem;
  font-weight: 600;
  padding: 2px 9px;
  border-radius: var(--radius-full);
  background-color: var(--accent-soft);
  color: var(--accent-primary);
  flex-shrink: 0;
}

.viewer-title {
  font-size: 0.98rem;
  font-weight: 600;
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
}

.viewer-header-actions {
  display: flex;
  align-items: center;
  gap: 8px;
  flex-shrink: 0;
}

.viewer-action-btn {
  height: 34px;
  padding: 0 11px;
  border-radius: var(--radius-sm);
  border: 1px solid var(--border-color);
  background-color: var(--bg-secondary);
  color: var(--text-secondary);
  font-size: 0.82rem;
  font-weight: 500;
  display: inline-flex;
  align-items: center;
  gap: 6px;
  transition: all var(--transition);
}

.viewer-action-btn:hover {
  background-color: var(--bg-tertiary);
  color: var(--text-primary);
}

.viewer-action-btn.close-btn:hover {
  background-color: rgba(239, 68, 68, 0.12);
  color: #ef4444;
  border-color: rgba(239, 68, 68, 0.3);
}

.viewer-body {
  flex: 1;
  position: relative;
  background-color: #ffffff;
}

.viewer-iframe {
  width: 100%;
  height: 100%;
  border: none;
  display: block;
}

/* ==========================================================================
   新增教材說明彈窗 (How to add modal)
   ========================================================================== */
.help-modal-content {
  max-width: 680px;
  height: auto;
  max-height: 85vh;
}

.help-body {
  padding: 24px 28px;
  overflow-y: auto;
  display: flex;
  flex-direction: column;
  gap: 18px;
  font-size: 0.92rem;
  color: var(--text-secondary);
}

.help-step {
  background-color: var(--bg-tertiary);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-md);
  padding: 16px 18px;
}

.help-step h4 {
  color: var(--text-primary);
  font-size: 0.98rem;
  margin-bottom: 6px;
}

.help-code {
  margin-top: 10px;
  padding: 12px 14px;
  border-radius: var(--radius-sm);
  background-color: #0f172a;
  color: #e2e8f0;
  font-family: ui-monospace, SFMono-Regular, Menlo, Monaco, Consolas, monospace;
  font-size: 0.8rem;
  overflow-x: auto;
  line-height: 1.5;
}

/* 頁尾 */
.site-footer {
  border-top: 1px solid var(--border-color);
  background-color: var(--bg-secondary);
  padding: 20px 24px;
  text-align: center;
  font-size: 0.84rem;
  color: var(--text-muted);
}

/* ==========================================================================
   響應式設計 (RWD)
   ========================================================================== */
@media (max-width: 992px) {
  .main-layout {
    grid-template-columns: 1fr;
    gap: 20px;
  }

  .sidebar {
    position: static;
  }

  .category-nav {
    flex-direction: row;
    overflow-x: auto;
    padding-bottom: 4px;
  }

  .category-btn {
    width: auto;
    flex-shrink: 0;
  }

  .guide-card {
    display: none;
  }
}

@media (max-width: 640px) {
  .header-inner {
    padding: 0 16px;
    gap: 12px;
  }

  .brand-subtitle,
  .search-shortcut,
  .btn-text-hide-mobile {
    display: none;
  }

  .category-banner {
    flex-direction: column;
    align-items: flex-start;
    padding: 20px;
  }

  .banner-stats {
    width: 100%;
    justify-content: flex-start;
  }

  .materials-grid.list-view .material-card {
    flex-direction: column;
    align-items: stretch;
  }

  .materials-grid.list-view .card-footer {
    flex-direction: row;
    align-items: center;
    border-top: 1px solid var(--border-color);
    padding-top: 12px;
  }

  .viewer-overlay {
    padding: 0;
  }

  .viewer-container {
    height: 100vh;
    border-radius: 0;
  }
}