/* ============================================
   RESPONSIVE — Mobile breakpoints
   ============================================ */

@media (max-width: 1024px) {
  .page-with-sidebar {
    grid-template-columns: 1fr;
  }

  .sidebar {
    position: static;
    max-height: none;
    display: flex;
    gap: var(--space-2);
    flex-wrap: wrap;
    padding-bottom: var(--space-4);
    border-bottom: 1px solid var(--border-color);
    margin-bottom: var(--space-4);
  }

  .sidebar__section {
    margin-bottom: 0;
  }

  .sidebar__section-title {
    display: none;
  }

  .topic-list {
    display: flex;
    gap: var(--space-1);
    flex-wrap: wrap;
  }

  .topic-list__item {
    margin-bottom: 0;
  }

  .track-topics-grid {
    grid-template-columns: 1fr;
  }
}

@media (max-width: 768px) {
  :root {
    --space-8: 1.5rem;
    --space-12: 2rem;
    --space-16: 3rem;
  }

  .nav__links {
    display: none;
    position: absolute;
    top: var(--nav-height);
    left: 0;
    right: 0;
    background: var(--bg-primary);
    border-bottom: 1px solid var(--border-color);
    flex-direction: column;
    padding: var(--space-4);
    gap: var(--space-1);
  }

  .nav__links--open {
    display: flex;
  }

  .nav__mobile-toggle {
    display: block;
  }

  .hero__title {
    font-size: var(--text-2xl);
  }

  .hero__subtitle {
    font-size: var(--text-base);
  }

  .hero__actions {
    flex-direction: column;
    align-items: center;
  }

  .stats-bar {
    gap: var(--space-6);
    flex-wrap: wrap;
  }

  .stat-item__number {
    font-size: var(--text-2xl);
  }

  .grid-2,
  .grid-3,
  .grid-4 {
    grid-template-columns: 1fr;
  }

  .track-cards {
    grid-template-columns: 1fr;
  }

  .filter-bar {
    flex-direction: column;
  }

  .filter-bar__search {
    min-width: auto;
    width: 100%;
  }

  .filter-bar__select {
    width: 100%;
  }

  .problem-card {
    flex-direction: column;
    align-items: flex-start;
    gap: var(--space-2);
  }

  .problem-card__tags {
    width: 100%;
  }

  .problem-detail__statement {
    padding: var(--space-4);
  }

  .footer__inner {
    flex-direction: column;
    gap: var(--space-4);
    text-align: center;
  }

  .section-header__title {
    font-size: var(--text-2xl);
  }
}

@media (max-width: 768px) {
  /* Toast: full-width on mobile instead of centered */
  .qr-toast {
    left: var(--space-3);
    right: var(--space-3);
    transform: translateY(20px) scale(0.95);
    text-align: center;
    white-space: normal;
    border-radius: var(--radius-lg);
  }
  .qr-toast--show {
    transform: translateY(0) scale(1);
  }
}

@media (max-width: 480px) {
  .nav__brand span:not(.nav__brand-icon) {
    display: none;
  }

  .lecture-viewer__content iframe {
    height: 50vh;
  }

  /* Hide level badge in nav to reduce crowding */
  .nav__user-level-badge {
    display: none;
  }

  /* Similar problems: stack vertically */
  .similar-problems {
    grid-template-columns: 1fr !important;
  }
}