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.

app/admin/components/AdminDashboard.tsx

@@ -49,6 +49,15 @@ export default function AdminDashboard({ onLogout }: Props) {
           </nav>
         </div>
         <div className="admin-header-right">
+          <a
+            href="/"
+            target="_blank"
+            rel="noopener noreferrer"
+            className="admin-btn admin-btn-ghost"
+            title="Open site in new tab"
+          >
+            ↗ Site
+          </a>
           <button
             className="admin-btn admin-btn-ghost"
             onClick={handleReload}