/* ── Sidebar Navigation ── */
  .sidebar {
    position: fixed;
    left: 0; top: 0;
    width: 260px;
    height: 100vh;
    background: var(--bg-surface-alt);
    border-right: 1px solid var(--border);
    padding: 20px 0 0;
    overflow-y: auto;
    z-index: 100;
    display: flex;
    flex-direction: column;
  }
  .sidebar-content { flex: 1; overflow-y: auto; }
  .sidebar-bottom {
    border-top: 1px solid var(--border);
    padding: 12px 16px;
    display: flex; flex-direction: column; gap: 8px;
    flex-shrink: 0;
  }
  .sidebar-bottom a, .sidebar-bottom select {
    font-family: var(--font);
  }
  .sb-back {
    display: flex; align-items: center; gap: 8px;
    padding: 10px 12px;
    background: var(--bg-inset); border: 1px solid var(--border); border-radius: 10px;
    color: var(--text-secondary); text-decoration: none; font-size: 13px; font-weight: 500;
    transition: all 0.2s;
  }
  .sb-back:hover { color: var(--concept-text); border-color: var(--concept); background: rgba(139,92,246,0.08); }
  .sb-select {
    padding: 10px 12px;
    background: var(--bg-inset); border: 1px solid var(--border); border-radius: 10px;
    color: var(--text-primary); font-size: 13px; font-weight: 500;
    cursor: pointer; appearance: none;
    background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='12' height='12' viewBox='0 0 24 24' fill='none' stroke='%23888' stroke-width='2'%3E%3Cpath d='M6 9l6 6 6-6'/%3E%3C/svg%3E");
    background-repeat: no-repeat;
    background-position: right 12px center;
    transition: border-color 0.2s;
  }
  .sb-select:hover { border-color: var(--concept); }
  .sb-select:focus { outline: none; border-color: var(--concept); }
  .sidebar h2 {
    color: var(--concept);
    font-size: 14px;
    padding: 0 20px;
    margin-bottom: 16px;
    letter-spacing: 2px;
    text-transform: uppercase;
  }
  .sidebar a {
    display: block;
    padding: 10px 20px;
    color: var(--text-secondary);
    text-decoration: none;
    font-size: 13px;
    border-left: 3px solid transparent;
    transition: all 0.2s;
  }
  .sidebar a:hover, .sidebar a.active {
    color: var(--text-primary);
    background: rgba(139, 92, 246, 0.1);
    border-left-color: var(--concept);
  }

  /* ── Sidebar Accordion ── */
  .nav-group { margin-bottom: 2px; }
  .nav-group .nav-links {
    max-height: 0;
    overflow: hidden;
    transition: max-height 0.3s ease;
  }
  .nav-group.open .nav-links {
    max-height: 500px;
  }
  .nav-arrow {
    display: inline-block;
    font-size: 10px;
    margin-right: 4px;
    transition: transform 0.2s ease;
  }
  .nav-group.open .nav-arrow {
    transform: rotate(90deg);
  }

  .sidebar .lecture-label {
    color: var(--concept);
    font-weight: 700;
    font-size: 12px;
    padding: 12px 20px 8px;
    letter-spacing: 1px;
    cursor: pointer;
    user-select: none;
    transition: background 0.2s;
  }
  .sidebar .lecture-label:hover {
    background: rgba(139, 92, 246, 0.1);
  }