feat(admin): add asset picker with favorites, drag-and-drop reordering, preview links, and keyboard shortcuts

- Asset Picker: Visual browser for hero images with Favorites tab (shown first)
  and All Photos tab. Includes infinite scroll and UUID fallback input for
  assets not easily found in the grid.
- Drag & Drop: All lists (hero images, standalone albums, subpage albums,
  subpage tiles) are now reorderable via drag handles using @dnd-kit.
- Preview Links: 'Preview Site' button in save bar, '↗ Site' in header, and
  per-subpage preview links with URL slug preview shown inline.
- Keyboard Shortcuts: ⌘+S / Ctrl+S to save in both Pages and Settings tabs.
- Unsaved Changes Guard: beforeunload warning prevents accidental data loss.
- New API route /api/admin/assets for browsing Immich assets with pagination
  and favorites filter via search/metadata endpoint.

package.json

@@ -15,6 +15,9 @@
     "test:e2e": "playwright test"
   },
   "dependencies": {
+    "@dnd-kit/core": "^6.3.1",
+    "@dnd-kit/sortable": "^10.0.0",
+    "@dnd-kit/utilities": "^3.2.2",
     "js-yaml": "^4.1.1",
     "leaflet": "^1.9.4",
     "next": "16.2.4",