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/components/AdminDashboard.tsx

@@ -0,0 +1,72 @@
+'use client';
+
+import { useState } from 'react';
+import PageBuilder from './PageBuilder';
+import SettingsEditor from './SettingsEditor';
+
+interface Props {
+  onLogout: () => void;
+}
+
+type Tab = 'pages' | 'settings';
+
+export default function AdminDashboard({ onLogout }: Props) {
+  const [activeTab, setActiveTab] = useState<Tab>('pages');
+  const [saving, setSaving] = useState(false);
+
+  async function handleLogout() {
+    await fetch('/api/admin/auth', { method: 'DELETE' });
+    onLogout();
+  }
+
+  async function handleReload() {
+    setSaving(true);
+    try {
+      await fetch('/api/admin/reload', { method: 'POST' });
+    } finally {
+      setSaving(false);
+    }
+  }
+
+  return (
+    <div className="admin-dashboard">
+      <header className="admin-header">
+        <div className="admin-header-left">
+          <h1>Immich Folio</h1>
+          <nav className="admin-tabs">
+            <button
+              className={`admin-tab ${activeTab === 'pages' ? 'active' : ''}`}
+              onClick={() => setActiveTab('pages')}
+            >
+              Pages
+            </button>
+            <button
+              className={`admin-tab ${activeTab === 'settings' ? 'active' : ''}`}
+              onClick={() => setActiveTab('settings')}
+            >
+              Settings
+            </button>
+          </nav>
+        </div>
+        <div className="admin-header-right">
+          <button
+            className="admin-btn admin-btn-ghost"
+            onClick={handleReload}
+            disabled={saving}
+            title="Reload config & clear cache"
+          >
+            ↻ Reload
+          </button>
+          <button className="admin-btn admin-btn-ghost" onClick={handleLogout}>
+            Logout
+          </button>
+        </div>
+      </header>
+
+      <main className="admin-main">
+        {activeTab === 'pages' && <PageBuilder />}
+        {activeTab === 'settings' && <SettingsEditor />}
+      </main>
+    </div>
+  );
+}