/* === CSS Variables === */
:root {
  --bg: #f5f7fa;
  --surface: #ffffff;
  --sidebar-bg: #1e293b;
  --sidebar-text: #e2e8f0;
  --accent: #3b82f6;
  --accent-hover: #2563eb;
  --text: #1e293b;
  --text-muted: #64748b;
  --border: #e2e8f0;
  --shadow: 0 4px 6px -1px rgba(0,0,0,0.1), 0 2px 4px -2px rgba(0,0,0,0.1);
  --radius: 12px;
  --transition: 0.2s ease;
}

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

html, body {
  height: 100%;
  font-family: 'Segoe UI', system-ui, -apple-system, sans-serif;
  background: var(--bg);
  color: var(--text);
  overflow: hidden;
}

#app {
  display: flex;
  height: 100vh;
  width: 100vw;
}

/* === Sidebar === */
#sidebar {
  width: 280px;
  background: var(--sidebar-bg);
  color: var(--sidebar-text);
  display: flex;
  flex-direction: column;
  flex-shrink: 0;
  transition: transform var(--transition);
}

.sidebar-header {
  padding: 20px;
  border-bottom: 1px solid rgba(255,255,255,0.1);
}

.sidebar-header h1 {
  font-size: 1.1rem;
  font-weight: 700;
  color: #fff;
  margin-bottom: 4px;
}

.subtitle {
  font-size: 0.75rem;
  color: #94a3b8;
}

.search-box {
  padding: 12px 16px;
}

.search-box input {
  width: 100%;
  padding: 8px 12px;
  border: 1px solid rgba(255,255,255,0.15);
  border-radius: 8px;
  background: rgba(255,255,255,0.08);
  color: #fff;
  font-size: 0.85rem;
  outline: none;
}

.search-box input::placeholder { color: #94a3b8; }
.search-box input:focus { border-color: var(--accent); }

#lesson-list {
  flex: 1;
  overflow-y: auto;
  padding: 0 8px 12px;
}

.lesson-item {
  display: flex;
  align-items: center;
  gap: 10px;
  padding: 10px 12px;
  border-radius: 8px;
  cursor: pointer;
  transition: background var(--transition);
  font-size: 0.85rem;
  margin-bottom: 2px;
}

.lesson-item:hover { background: rgba(255,255,255,0.08); }
.lesson-item.active {
  background: var(--accent);
  color: #fff;
}

.lesson-num {
  width: 28px;
  height: 28px;
  display: flex;
  align-items: center;
  justify-content: center;
  background: rgba(255,255,255,0.12);
  border-radius: 6px;
  font-weight: 700;
  font-size: 0.75rem;
  flex-shrink: 0;
}

.lesson-item.active .lesson-num { background: rgba(255,255,255,0.25); }

.lesson-title { line-height: 1.3; }
.lesson-title small {
  display: block;
  color: #94a3b8;
  font-size: 0.75rem;
}
.lesson-item.active .lesson-title small { color: rgba(255,255,255,0.75); }

.sidebar-footer {
  padding: 12px 16px;
  border-top: 1px solid rgba(255,255,255,0.1);
}

.btn {
  width: 100%;
  padding: 10px;
  border: none;
  border-radius: 8px;
  background: var(--accent);
  color: #fff;
  font-weight: 600;
  cursor: pointer;
  transition: background var(--transition);
}

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

/* === Main === */
#main {
  flex: 1;
  display: flex;
  flex-direction: column;
  overflow: hidden;
  position: relative;
}

#top-bar {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 10px 20px;
  background: var(--surface);
  border-bottom: 1px solid var(--border);
  box-shadow: var(--shadow);
  z-index: 10;
}

.breadcrumb {
  font-size: 0.9rem;
  color: var(--text-muted);
}

.breadcrumb .sep { margin: 0 6px; color: var(--border); }

#current-lesson { font-weight: 600; color: var(--text); }

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

.icon-btn {
  width: 34px;
  height: 34px;
  border: 1px solid var(--border);
  border-radius: 8px;
  background: var(--surface);
  cursor: pointer;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 0.9rem;
  transition: all var(--transition);
}

.icon-btn:hover {
  border-color: var(--accent);
  color: var(--accent);
}

#page-input {
  width: 56px;
  text-align: center;
  padding: 6px;
  border: 1px solid var(--border);
  border-radius: 6px;
  font-size: 0.9rem;
  outline: none;
}

#page-input:focus { border-color: var(--accent); }

/* === Viewer === */
#viewer {
  flex: 1;
  overflow: auto;
  padding: 20px;
  background: #e2e8f0;
  cursor: default;
  -webkit-user-select: none;
  user-select: none;
}
#viewer.grabbing { cursor: grabbing; }
#viewer.grab { cursor: grab; }

#page-container {
  display: inline-block;
  background: var(--surface);
  border-radius: var(--radius);
  box-shadow: 0 20px 25px -5px rgba(0,0,0,0.1), 0 8px 10px -6px rgba(0,0,0,0.1);
  overflow: hidden;
  vertical-align: top;
}

#page-image {
  display: block;
  width: 100%;
  height: auto;
  transition: width 0.2s, height 0.2s;
  -webkit-user-drag: none;
  user-drag: none;
  -webkit-user-select: none;
  user-select: none;
}

/* === Audio Panel === */
#audio-panel {
  width: 320px;
  background: var(--surface);
  border-left: 1px solid var(--border);
  display: flex;
  flex-direction: column;
  transition: transform var(--transition);
}

#audio-panel.hidden { display: none; }

.audio-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 16px;
  border-bottom: 1px solid var(--border);
}

.audio-header h2 { font-size: 1rem; }

.audio-player-wrap {
  padding: 16px;
  border-bottom: 1px solid var(--border);
}

#audio-player {
  width: 100%;
  border-radius: 8px;
}

.now-playing {
  margin-top: 8px;
  font-size: 0.8rem;
  color: var(--text-muted);
  text-align: center;
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
}

.audio-tabs {
  display: flex;
  padding: 12px 16px 0;
  gap: 8px;
}

.tab-btn {
  flex: 1;
  padding: 8px;
  border: 1px solid var(--border);
  border-radius: 8px;
  background: var(--bg);
  cursor: pointer;
  font-weight: 600;
  font-size: 0.85rem;
  transition: all var(--transition);
}

.tab-btn.active {
  background: var(--accent);
  color: #fff;
  border-color: var(--accent);
}

#track-list {
  flex: 1;
  overflow-y: auto;
  padding: 8px 16px 16px;
}

.track-item {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 8px 10px;
  border-radius: 8px;
  cursor: pointer;
  transition: background var(--transition);
  font-size: 0.85rem;
  margin-bottom: 2px;
}

.track-item:hover { background: var(--bg); }
.track-item.active {
  background: var(--accent);
  color: #fff;
}

.track-item .track-num { font-weight: 700; width: 28px; }
.track-item .track-name { flex: 1; }
.track-item .track-play { font-size: 0.8rem; }

.suggested-box {
  margin: 0 16px 8px;
  padding: 10px 12px;
  background: #f0f9ff;
  border: 1px solid #bae6fd;
  border-radius: 8px;
  font-size: 0.8rem;
}
.suggested-box h4 {
  font-size: 0.75rem;
  color: #0369a1;
  margin-bottom: 6px;
  text-transform: uppercase;
  letter-spacing: 0.5px;
}
.suggested-tracks {
  display: flex;
  flex-wrap: wrap;
  gap: 6px;
}
.suggested-chip {
  display: inline-flex;
  align-items: center;
  gap: 4px;
  padding: 4px 10px;
  background: #fff;
  border: 1px solid #7dd3fc;
  border-radius: 20px;
  cursor: pointer;
  font-size: 0.8rem;
  transition: all var(--transition);
}
.suggested-chip:hover {
  background: var(--accent);
  color: #fff;
  border-color: var(--accent);
}
.suggested-chip .cd-label {
  font-size: 0.65rem;
  opacity: 0.7;
}
::-webkit-scrollbar { width: 6px; }
::-webkit-scrollbar-track { background: transparent; }
::-webkit-scrollbar-thumb { background: #cbd5e1; border-radius: 3px; }
::-webkit-scrollbar-thumb:hover { background: #94a3b8; }

.sidebar-top-row {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 10px;
}

.sidebar-close-btn {
  display: none;
  width: 32px; height: 32px;
  border: 1px solid rgba(255,255,255,0.2);
  border-radius: 8px;
  background: rgba(255,255,255,0.1);
  color: #fff;
  cursor: pointer;
  align-items: center;
  justify-content: center;
}

@media (max-width: 900px) {
  .sidebar-close-btn { display: flex; }
}

/* === Overlay === */
.overlay {
  display: none;
  position: fixed;
  inset: 0;
  background: rgba(0,0,0,0.45);
  z-index: 90;
  opacity: 0;
  transition: opacity var(--transition);
}
.overlay.active { display: block; opacity: 1; }

/* === Responsive === */
@media (max-width: 900px) {
  #sidebar {
    position: fixed;
    left: 0; top: 0;
    width: 280px;
    height: 100%;
    z-index: 100;
    transform: translateX(-100%);
  }
  #sidebar.open { transform: translateX(0); }

  #audio-panel {
    position: fixed;
    left: 0; right: 0;
    bottom: 0;
    top: auto;
    width: 100%;
    height: 65vh;
    max-height: 520px;
    border-left: none;
    border-top: 1px solid var(--border);
    border-radius: 16px 16px 0 0;
    z-index: 100;
    transform: translateY(100%);
  }
  #audio-panel.open { transform: translateY(0); }
  #audio-panel.hidden { display: flex; transform: translateY(100%); }
  #audio-panel:not(.hidden) { transform: translateY(0); }

  .audio-player-wrap {
    padding: 12px 16px;
  }
  #audio-player { height: 44px; }
  .track-item { padding: 12px 10px; font-size: 1rem; }
  .tab-btn { padding: 10px; font-size: 1rem; }
  .suggested-chip { padding: 6px 14px; font-size: 1rem; }

  #mobile-menu-btn { display: block; }
  #top-bar { padding-left: 56px; }
}

#mobile-menu-btn {
  display: none;
  position: absolute;
  left: 10px; top: 10px; z-index: 50;
  width: 40px; height: 40px;
  border-radius: 8px; border: 1px solid var(--border);
  background: var(--surface); font-size: 1.2rem; cursor: pointer;
}

@media (max-width: 900px) {
  #mobile-menu-btn { display: block; }
  #top-bar { padding-left: 56px; }
}