/* Original lesson-specific/demo styles extracted from index.html; consolidate after visual QA. */

    /* 1. Pretendard WebFont Full Weights */
    @font-face {
      font-family: 'Pretendard';
      src: url('https://cdn.jsdelivr.net/gh/projectnoonnu/pretendard@1.0/Pretendard-Thin.woff2') format('woff2');
      font-weight: 100;
      font-display: swap;
    }
    @font-face {
      font-family: 'Pretendard';
      src: url('https://cdn.jsdelivr.net/gh/projectnoonnu/pretendard@1.0/Pretendard-ExtraLight.woff2') format('woff2');
      font-weight: 200;
      font-display: swap;
    }
    @font-face {
      font-family: 'Pretendard';
      src: url('https://cdn.jsdelivr.net/gh/projectnoonnu/pretendard@1.0/Pretendard-Light.woff2') format('woff2');
      font-weight: 300;
      font-display: swap;
    }
    @font-face {
      font-family: 'Pretendard';
      src: url('https://cdn.jsdelivr.net/gh/projectnoonnu/pretendard@1.0/Pretendard-Regular.woff2') format('woff2');
      font-weight: 400;
      font-display: swap;
    }
    @font-face {
      font-family: 'Pretendard';
      src: url('https://cdn.jsdelivr.net/gh/projectnoonnu/pretendard@1.0/Pretendard-Medium.woff2') format('woff2');
      font-weight: 500;
      font-display: swap;
    }
    @font-face {
      font-family: 'Pretendard';
      src: url('https://cdn.jsdelivr.net/gh/projectnoonnu/pretendard@1.0/Pretendard-SemiBold.woff2') format('woff2');
      font-weight: 600;
      font-display: swap;
    }
    @font-face {
      font-family: 'Pretendard';
      src: url('https://cdn.jsdelivr.net/gh/projectnoonnu/pretendard@1.0/Pretendard-Bold.woff2') format('woff2');
      font-weight: 700;
      font-display: swap;
    }
    @font-face {
      font-family: 'Pretendard';
      src: url('https://cdn.jsdelivr.net/gh/projectnoonnu/pretendard@1.0/Pretendard-ExtraBold.woff2') format('woff2');
      font-weight: 800;
      font-display: swap;
    }
    @font-face {
      font-family: 'Pretendard';
      src: url('https://cdn.jsdelivr.net/gh/projectnoonnu/pretendard@1.0/Pretendard-Black.woff2') format('woff2');
      font-weight: 900;
      font-display: swap;
    }

    /* 2. Theme Tokens (Light & Dark) */
    :root {
      --bg: #f8f9fa;
      --surface: #ffffff;
      --surface-subtle: #f1f3f5;
      --surface-hover: #ffffff;
      
      --border: #e9ecef;
      --border-hover: #ced4da;
      --border-focus: #111827;
      
      --text: #111827;
      --text-sub: #4b5563;
      --text-muted: #868e96;
      
      --nav-bg: rgba(248, 249, 250, 0.85);
      --card-bg: rgba(255, 255, 255, 0.9);
      
      --tag-bg: #f1f3f5;
      --tag-text: #495057;
      --tag-quiz-bg: #fef3c7;
      --tag-quiz-text: #92400e;
      --tag-lecture-bg: #e6fcf5;
      --tag-lecture-text: #0ca678;
    }

    

    /* 3. Base Reset */


    html {
      font-family: 'Pretendard', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
      font-size: 16px;
      scroll-behavior: smooth;
      -webkit-font-smoothing: antialiased;
      -moz-osx-font-smoothing: grayscale;
    }

    body {
      font-family: 'Pretendard', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
      background-color: var(--bg);
      color: var(--text);
      line-height: 1.6;
      letter-spacing: -0.02em;
      min-height: 100vh;
      transition: background-color 0.2s ease, color 0.2s ease;
    }

    /* 4. Accessibility Skip Link */
    .skip-link {
      position: fixed;
      top: -100px;
      left: 30px;
      padding: 8px 18px;
      min-height: 30px;
      display: inline-flex;
      align-items: center;
      background: var(--text);
      color: var(--bg);
      border-radius: 6px;
      font-weight: 600;
      font-size: 14px;
      z-index: 99999;
      text-decoration: none;
      transition: top 0.2s ease;
    }
    .skip-link:focus {
      top: 20px;
      outline: 2px solid var(--border-focus);
    }

    /* 5. Sticky Clean Header */
    .header-nav {
      position: sticky;
      top: 0;
      left: 0;
      right: 0;
      height: 64px;
      background: var(--nav-bg);
      backdrop-filter: blur(16px);
      -webkit-backdrop-filter: blur(16px);
      border-bottom: 1px solid var(--border);
      z-index: 1000;
      display: flex;
      align-items: center;
      padding: 0 30px;
    }

    .nav-container {
      max-width: 1080px;
      width: 100%;
      margin: 0 auto;
      display: flex;
      align-items: center;
      justify-content: space-between;
    }

    .brand-group {
      display: flex;
      align-items: center;
      gap: 12px;
    }

    .brand-title {
      font-size: 18px;
      font-weight: 700;
      color: var(--text);
      text-decoration: none;
      letter-spacing: -0.03em;
    }

    .brand-badge {
      font-size: 12px;
      font-weight: 500;
      color: var(--text-muted);
      border: 1px solid var(--border);
      padding: 4px 12px;
      min-height: 30px;
      display: inline-flex;
      align-items: center;
      justify-content: center;
      border-radius: 6px;
    }

    .nav-links {
      display: none;
      align-items: center;
      gap: 20px;
    }

    @media (min-width: 768px) {
      .nav-links {
        display: flex;
      }
    }

    .nav-link {
      font-size: 15px;
      font-weight: 500;
      color: var(--text-sub);
      text-decoration: none;
      min-height: 30px;
      display: inline-flex;
      align-items: center;
      transition: color 0.15s ease;
    }
    .nav-link:hover {
      color: var(--text);
    }

    .nav-actions {
      display: flex;
      align-items: center;
      gap: 12px;
    }

    

    /* 6. Minimal Hero */
    .hero {
      padding: 60px 30px 48px;
      text-align: center;
      background: var(--surface);
      border-bottom: 1px solid var(--border);
    }

    .hero-inner {
      max-width: 860px;
      margin: 0 auto;
      display: flex;
      flex-direction: column;
      align-items: center;
      gap: 12px;
    }

    .hero-title {
      font-size: clamp(30px, 4.5vw, 40px);
      font-weight: 800;
      letter-spacing: -0.04em;
      color: var(--text);
      line-height: 1.2;
    }

    .hero-desc {
      font-size: 16px;
      font-weight: 400;
      color: var(--text-sub);
      line-height: 1.6;
    }

    .hero-badge {
      display: inline-flex;
      align-items: center;
      justify-content: center;
      background: var(--surface-subtle);
      border: 1px solid var(--border);
      color: var(--text-sub);
      font-size: 13px;
      font-weight: 500;
      padding: 4px 16px;
      min-height: 30px;
      border-radius: 20px;
      margin-top: 6px;
    }

    /* 7. Main Wrapper & Section Blocks */
    .main-wrapper {
      max-width: 1080px;
      margin: 0 auto;
      padding: 48px 30px 80px;
    }

    .section-block {
      margin-bottom: 52px;
    }

    .section-label {
      font-size: 13px;
      font-weight: 700;
      letter-spacing: 0.08em;
      color: var(--text-muted);
      text-transform: uppercase;
      margin-bottom: 30px;
      display: flex;
      align-items: center;
      gap: 16px;
    }

    .section-label::after {
      content: '';
      flex: 1;
      height: 1px;
      background: var(--border);
    }

    /* 8. Auto-Layout Grid (Responsive) - 간격 30px 이상 */
    .card-grid {
      display: grid;
      grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
      gap: 30px;
    }

    @media (min-width: 900px) {
      .card-grid {
        grid-template-columns: repeat(3, 1fr);
      }
    }

    /* 9. Clean Cards - 내부 여백(padding) 30px 이상 */
    .card {
      background: var(--card-bg);
      border: 1px solid var(--border);
      border-radius: 14px;
      padding: 30px;
      text-decoration: none;
      color: var(--text);
      display: flex;
      flex-direction: column;
      justify-content: space-between;
      gap: 16px;
      transition: border-color 0.15s ease, background-color 0.15s ease;
    }

    .card:hover {
      border-color: var(--border-hover);
      background-color: var(--surface-hover);
    }

    .card-top {
      display: flex;
      align-items: center;
      justify-content: space-between;
      min-height: 32px;
    }

    /* 카드 아이콘 크기 (32px: 30px 이상) */
    .card-icon {
      font-size: 32px;
      line-height: 1;
      min-width: 32px;
      min-height: 32px;
      display: inline-flex;
      align-items: center;
      justify-content: center;
    }

    .card-body {
      display: flex;
      flex-direction: column;
      gap: 8px;
    }

    .card-title {
      font-size: 18px;
      font-weight: 700;
      letter-spacing: -0.02em;
      color: var(--text);
      line-height: 1.35;
    }

    .card-desc {
      font-size: 14px;
      color: var(--text-sub);
      line-height: 1.6;
    }

    .card-tags {
      display: flex;
      flex-wrap: wrap;
      gap: 8px;
      padding-top: 16px;
      border-top: 1px solid var(--border);
    }

    /* 태그 최소 높이 (30px 이상) */
    .tag {
      font-size: 12px;
      text-decoration: none;
      font-weight: 500;
      padding: 4px 12px;
      min-height: 30px;
      display: inline-flex;
      align-items: center;
      justify-content: center;
      border-radius: 6px;
      background: var(--tag-bg);
      color: var(--tag-text);
    }

    .tag.lecture {
      background: var(--tag-lecture-bg);
      color: var(--tag-lecture-text);
    }

    .tag.quiz {
      background: var(--tag-quiz-bg);
      color: var(--tag-quiz-text);
    }

    /* Soon Card */
    .card.soon {
      opacity: 0.55;
      pointer-events: none;
    }

    /* 준비중 뱃지 (30px 이상) */
    .badge-soon {
      font-size: 12px;
      font-weight: 600;
      color: var(--text-muted);
      background: var(--tag-bg);
      padding: 4px 12px;
      min-height: 30px;
      display: inline-flex;
      align-items: center;
      justify-content: center;
      border-radius: 6px;
    }

    /* 10. Footer - 좌우/상하 패딩 30px 이상 */
    .site-footer {
      border-top: 1px solid var(--border);
      padding: 40px 30px;
      text-align: center;
      font-size: 14px;
      color: var(--text-muted);
      background: var(--surface);
    }