feat: add visual admin panel with page builder and settings editor

- Add /admin route with password-protected visual page builder
- Page Builder: manage hero images, standalone albums, subpages, and sections
- Album Picker: browse all shared Immich albums with search and filtering
- Settings Editor: theme, grid, footer, legal, SEO configuration
- YAML service with atomic writes and automatic backups (content/.backups/)
- HMAC-signed admin sessions (24h expiry, HttpOnly cookies)
- API endpoints: /api/admin/{auth,gallery,settings,albums,reload}
- Enable with ADMIN_PASSWORD env var; disabled by default
- invalidateConfigCache() export for hot-reload after saves

app/admin/layout.tsx

@@ -0,0 +1,10 @@
+import type { Metadata } from 'next';
+
+export const metadata: Metadata = {
+  title: 'Immich Folio Admin',
+  robots: { index: false, follow: false },
+};
+
+export default function AdminLayout({ children }: { children: React.ReactNode }) {
+  return <div className="admin-layout">{children}</div>;
+}