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/page.tsx

@@ -0,0 +1,54 @@
+'use client';
+
+import { useState, useEffect } from 'react';
+import AdminLogin from './components/AdminLogin';
+import AdminDashboard from './components/AdminDashboard';
+import './admin.css';
+
+export default function AdminPage() {
+  const [authenticated, setAuthenticated] = useState<boolean | null>(null);
+  const [enabled, setEnabled] = useState(true);
+
+  useEffect(() => {
+    checkAuth();
+  }, []);
+
+  async function checkAuth() {
+    try {
+      const res = await fetch('/api/admin/auth');
+      const data = await res.json();
+      setAuthenticated(data.authenticated);
+      setEnabled(data.enabled);
+    } catch {
+      setAuthenticated(false);
+    }
+  }
+
+  if (authenticated === null) {
+    return (
+      <div className="admin-loading">
+        <div className="admin-spinner" />
+      </div>
+    );
+  }
+
+  if (!enabled) {
+    return (
+      <div className="admin-disabled">
+        <div className="admin-disabled-card">
+          <h1>Admin Panel</h1>
+          <p>
+            The admin panel is not enabled. Set <code>ADMIN_PASSWORD</code> in your environment
+            variables to activate it.
+          </p>
+        </div>
+      </div>
+    );
+  }
+
+  if (!authenticated) {
+    return <AdminLogin onSuccess={() => setAuthenticated(true)} />;
+  }
+
+  return <AdminDashboard onLogout={() => setAuthenticated(false)} />;
+}