Add QoL features: preview theme, external resources, shortcuts, mobile layout

- Dark/light preview theme toggle with localStorage persistence and
  dark CSS injection in preview, export, and embed
- External CSS/JS resources modal with per-fiddle persistence in
  options column, injected as link/script tags
- Keyboard shortcuts cheat sheet modal (? button or ? key)
- Mobile-responsive CSS with breakpoints at 768px and 480px
  for both editor and browse pages

public/css/browse.css

@@ -193,3 +193,16 @@ html, body {
   color: var(--text-dim);
   font-size: 14px;
 }
+
+/* Mobile responsive */
+@media (max-width: 768px) {
+  .browse-header { padding: 10px 16px; }
+  .browse-toolbar { padding: 10px 16px; }
+  .tags-bar { padding: 0 16px 10px; }
+  .fiddle-grid { grid-template-columns: 1fr; padding: 0 16px 16px; gap: 12px; }
+  .pagination { padding: 0 16px 16px; }
+}
+@media (max-width: 480px) {
+  .fiddle-grid { grid-template-columns: 1fr; }
+  #search-input { min-width: 0; }
+}