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/api/admin/assets/route.ts

@@ -0,0 +1,87 @@
+/**
+ * Admin API: Browse Immich assets for the hero picker.
+ * Supports pagination and fetching favorites first.
+ */
+
+import { NextRequest, NextResponse } from 'next/server';
+import { isAdminAuthenticated, isAdminEnabled } from '@/lib/admin/auth';
+import { getConfig } from '@/lib/config';
+
+interface ImmichSearchResult {
+  assets: {
+    items: Array<{
+      id: string;
+      type: string;
+      originalFileName: string;
+      thumbhash: string | null;
+      fileCreatedAt: string;
+      isFavorite: boolean;
+    }>;
+    nextPage: string | null;
+  };
+}
+
+/** GET: Search/browse assets from Immich for hero image selection. */
+export async function GET(request: NextRequest) {
+  if (!isAdminEnabled()) {
+    return NextResponse.json({ error: 'Admin not enabled' }, { status: 403 });
+  }
+  if (!(await isAdminAuthenticated())) {
+    return NextResponse.json({ error: 'Unauthorized' }, { status: 401 });
+  }
+
+  const config = getConfig();
+  if (config.needsSetup) {
+    return NextResponse.json({ error: 'Immich not configured' }, { status: 503 });
+  }
+
+  const { searchParams } = new URL(request.url);
+  const page = parseInt(searchParams.get('page') || '1', 10);
+  const favoritesOnly = searchParams.get('favorites') === 'true';
+  const pageSize = 50;
+
+  try {
+    // Use Immich search/metadata endpoint to query assets
+    const body: Record<string, unknown> = {
+      type: 'IMAGE',
+      size: pageSize,
+      page,
+      order: 'desc',
+      isNotInAlbum: false,
+    };
+
+    if (favoritesOnly) {
+      body.isFavorite = true;
+    }
+
+    const res = await fetch(`${config.immich.apiUrl}/search/metadata`, {
+      method: 'POST',
+      headers: {
+        'x-api-key': config.immich.apiKey,
+        'Content-Type': 'application/json',
+        Accept: 'application/json',
+      },
+      body: JSON.stringify(body),
+    });
+
+    if (!res.ok) {
+      return NextResponse.json({ error: `Immich API returned ${res.status}` }, { status: 502 });
+    }
+
+    const data = (await res.json()) as ImmichSearchResult;
+    const assets = data.assets.items.map((asset) => ({
+      id: asset.id,
+      originalFileName: asset.originalFileName,
+      fileCreatedAt: asset.fileCreatedAt,
+      isFavorite: asset.isFavorite,
+    }));
+
+    return NextResponse.json({
+      assets,
+      nextPage: data.assets.nextPage ? page + 1 : null,
+    });
+  } catch (err) {
+    console.error('[Admin] Failed to fetch assets from Immich:', err);
+    return NextResponse.json({ error: 'Failed to connect to Immich' }, { status: 502 });
+  }
+}