/* ── View Toggle ── */
  .view-hero, .view-lecture-1, .view-lecture-2, .view-lecture-3, .view-lecture-4, .view-lecture-5, .view-lecture-6, .view-lecture-7, .view-lecture-8, .view-exam, .view-quiz-1, .view-quiz-2, .view-quiz-3, .view-quiz-4, .view-quiz-5, .view-quiz-6, .view-quiz-7, .view-quiz-8, .view-calc, .view-ztable, .view-ttable, .view-vocab { display: none; }
  .view-hero.active, .view-lecture-1.active, .view-lecture-2.active, .view-lecture-3.active, .view-lecture-4.active, .view-lecture-5.active, .view-lecture-6.active, .view-lecture-7.active, .view-lecture-8.active, .view-exam.active, .view-quiz-1.active, .view-quiz-2.active, .view-quiz-3.active, .view-quiz-4.active, .view-quiz-5.active, .view-quiz-6.active, .view-quiz-7.active, .view-quiz-8.active, .view-calc.active, .view-ztable.active, .view-ttable.active, .view-vocab.active { display: block; animation: viewIn 0.3s ease; }
  .view-hero.active { display: block; }
  @keyframes viewIn {
    from { opacity: 0; transform: translateY(16px); }
    to { opacity: 1; transform: translateY(0); }
  }

  /* ── Quiz Section ── */
  .quiz-hero {
    text-align: center;
    padding: 40px 30px;
    margin-bottom: 32px;
    background: linear-gradient(135deg, rgba(59,130,246,0.08), rgba(139,92,246,0.08));
    border: 1px solid rgba(59,130,246,0.2);
    border-radius: var(--radius-xl);
    position: relative;
    overflow: hidden;
  }
  .quiz-hero::before {
    content: '';
    position: absolute;
    top: -50%; left: -50%;
    width: 200%; height: 200%;
    background: radial-gradient(circle at 30% 50%, rgba(59,130,246,0.06) 0%, transparent 50%),
                radial-gradient(circle at 70% 50%, rgba(139,92,246,0.06) 0%, transparent 50%);
    animation: quizGlow 8s ease-in-out infinite alternate;
  }
  .quiz-hero h3 {
    font-size: 22px; font-weight: 700; color: var(--text-white);
    margin-bottom: 6px; position: relative;
  }
  .quiz-hero p { color: var(--text-secondary); font-size: 14px; position: relative; }

  .quiz-card {
    background: var(--bg-surface); border: 1px solid var(--border);
    border-radius: var(--radius-xl); padding: 0; margin-bottom: 28px;
    transition: border-color 0.3s, box-shadow 0.3s; overflow: hidden;
  }
  .quiz-card:hover {
    border-color: rgba(139,92,246,0.4);
    box-shadow: 0 4px 24px rgba(139,92,246,0.08);
  }
  .quiz-card-header {
    display: flex; align-items: center; gap: 14px; padding: 22px 28px 0;
  }
  .quiz-num {
    display: flex; align-items: center; justify-content: center;
    width: 40px; height: 40px; border-radius: var(--radius-md);
    font-weight: 900; font-size: 16px; color: var(--text-white); flex-shrink: 0;
  }
  .quiz-num.q-obj { background: linear-gradient(135deg, var(--concept), var(--concept-dark)); box-shadow: 0 4px 12px rgba(139,92,246,0.3); }
  .quiz-num.q-subj { background: linear-gradient(135deg, var(--formula), #2563eb); box-shadow: 0 4px 12px rgba(59,130,246,0.3); }
  .quiz-type {
    display: inline-flex; align-items: center; gap: 5px;
    padding: 4px 14px; border-radius: 20px;
    font-size: 12px; font-weight: 700; letter-spacing: 0.5px;
  }
  .quiz-type.obj { background: rgba(139,92,246,0.12); color: var(--concept-text); border: 1px solid rgba(139,92,246,0.25); }
  .quiz-type.subj { background: rgba(59,130,246,0.12); color: var(--formula-text); border: 1px solid rgba(59,130,246,0.25); }
  .quiz-card-body { padding: 20px 28px 28px; }
  .quiz-question { font-size: 16px; font-weight: 500; color: var(--text-primary); margin-bottom: 20px; line-height: 1.9; }

  .quiz-options { list-style: none; padding: 0; margin: 0 0 8px; }
  .quiz-options li {
    display: flex; align-items: center; gap: 14px;
    padding: 14px 18px; margin: 8px 0;
    background: var(--bg-inset); border: 2px solid transparent; border-radius: 14px;
    cursor: pointer; transition: all 0.25s cubic-bezier(0.4, 0, 0.2, 1);
    font-size: 14px; line-height: 1.6;
  }
  .quiz-options li::before {
    content: attr(data-label);
    display: flex; align-items: center; justify-content: center;
    width: 30px; height: 30px; border-radius: 50%;
    background: var(--border); color: var(--text-secondary);
    font-weight: 700; font-size: 13px; flex-shrink: 0; transition: all 0.25s;
  }
  .quiz-options li:hover { background: rgba(139,92,246,0.06); border-color: rgba(139,92,246,0.3); transform: translateX(4px); }
  .quiz-options li:hover::before { background: rgba(139,92,246,0.15); color: var(--concept); }
  .quiz-options li.selected { border-color: var(--concept); background: rgba(139,92,246,0.1); }
  .quiz-options li.selected::before { background: var(--concept); color: var(--text-white); }
  .quiz-options li.correct { border-color: var(--example); background: rgba(34,197,94,0.08); cursor: default; }
  .quiz-options li.correct::before { background: var(--example); color: var(--text-white); content: '\2713'; }
  .quiz-options li.wrong { border-color: var(--error); background: rgba(239,68,68,0.08); cursor: default; }
  .quiz-options li.wrong::before { background: var(--error); color: var(--text-white); content: '\2717'; }
  .quiz-options[data-submitted] li { cursor: default; }
  .quiz-options[data-submitted] li:hover { transform: none; }

  .quiz-textarea {
    width: 100%; min-height: 90px; background: var(--bg-inset);
    border: 2px solid var(--border); border-radius: 14px; color: var(--text-primary);
    padding: 16px 18px; font-family: var(--font);
    font-size: 14px; line-height: 1.7; resize: vertical;
    transition: border-color 0.3s, box-shadow 0.3s;
  }
  .quiz-textarea:focus { outline: none; border-color: var(--formula); box-shadow: 0 0 0 3px rgba(59,130,246,0.15); }
  .quiz-textarea[readonly] { opacity: 0.7; cursor: default; }

  .quiz-hint-btn {
    display: inline-flex; align-items: center; gap: 6px;
    margin-top: 16px; margin-right: 10px; padding: 10px 20px;
    background: transparent; color: var(--tip); border: 1px solid var(--tip);
    border-radius: var(--radius-md); font-family: var(--font);
    font-size: 13px; font-weight: 600; cursor: pointer;
    transition: all 0.2s;
  }
  .quiz-hint-btn:hover { background: rgba(234,179,8,0.1); }
  .quiz-hint {
    margin-top: 12px; padding: 16px 20px;
    background: linear-gradient(135deg, rgba(234,179,8,0.06), rgba(234,179,8,0.02));
    border-left: 4px solid var(--tip); border-radius: 0 var(--radius-md) var(--radius-md) 0;
    display: none; font-size: 14px; line-height: 1.8; color: var(--tip-text);
    animation: quizFadeIn 0.3s ease;
  }
  .quiz-hint.show { display: block; }
  .quiz-hint strong { color: var(--tip); }

  .quiz-submit {
    display: inline-flex; align-items: center; gap: 8px;
    margin-top: 16px; padding: 12px 32px;
    background: linear-gradient(135deg, var(--concept), var(--concept-dark)); color: var(--text-white); border: none;
    border-radius: var(--radius-md); font-family: var(--font);
    font-size: 14px; font-weight: 700; cursor: pointer;
    transition: all 0.2s cubic-bezier(0.4, 0, 0.2, 1);
    box-shadow: 0 4px 14px rgba(139,92,246,0.25);
  }
  .quiz-submit:hover { transform: translateY(-2px); box-shadow: 0 6px 20px rgba(139,92,246,0.4); }
  .quiz-submit:active { transform: translateY(0); }
  .quiz-submit:disabled { opacity: 0.4; cursor: not-allowed; transform: none; box-shadow: none; }

  .quiz-reset {
    display: inline-flex; align-items: center; gap: 8px;
    margin-top: 16px; margin-left: 10px; padding: 12px 24px;
    background: transparent; color: var(--text-secondary); border: 1px solid var(--border);
    border-radius: var(--radius-md); font-family: var(--font);
    font-size: 14px; font-weight: 600; cursor: pointer;
    transition: all 0.2s;
  }
  .quiz-reset:hover { background: var(--bg-inset); color: var(--text-primary); border-color: var(--text-secondary); }

  .quiz-result {
    margin-top: 16px; padding: 14px 20px; border-radius: 14px;
    font-weight: 600; font-size: 15px; display: none;
    animation: quizFadeIn 0.4s ease;
  }
  @keyframes quizFadeIn {
    from { opacity: 0; transform: translateY(-8px); }
    to { opacity: 1; transform: translateY(0); }
  }
  .quiz-result.correct {
    display: flex; align-items: center; gap: 10px;
    background: linear-gradient(135deg, rgba(34,197,94,0.1), rgba(34,197,94,0.05));
    border: 1px solid rgba(34,197,94,0.3); color: var(--example-text);
  }
  .quiz-result.wrong {
    display: flex; align-items: center; gap: 10px;
    background: linear-gradient(135deg, rgba(239,68,68,0.1), rgba(239,68,68,0.05));
    border: 1px solid rgba(239,68,68,0.3); color: var(--error-text);
  }

  .quiz-explanation {
    margin-top: 16px; padding: 20px 24px;
    background: var(--concept-bg);
    border-left: 4px solid var(--concept); border-radius: 0 14px 14px 0;
    display: none; font-size: 14px; line-height: 1.9; color: var(--text-primary);
    animation: quizFadeIn 0.5s ease;
  }
  .quiz-explanation.show { display: block; }
  .quiz-explanation strong { color: var(--concept); }
  .quiz-explain-label {
    display: inline-flex; align-items: center; gap: 6px;
    font-size: 13px; font-weight: 700; color: var(--concept);
    text-transform: uppercase; letter-spacing: 1px; margin-bottom: 10px;
  }