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/AdminLogin.tsx

@@ -0,0 +1,68 @@
+'use client';
+
+import { useState } from 'react';
+
+interface Props {
+  onSuccess: () => void;
+}
+
+export default function AdminLogin({ onSuccess }: Props) {
+  const [password, setPassword] = useState('');
+  const [error, setError] = useState('');
+  const [loading, setLoading] = useState(false);
+
+  async function handleSubmit(e: React.FormEvent) {
+    e.preventDefault();
+    setError('');
+    setLoading(true);
+
+    try {
+      const res = await fetch('/api/admin/auth', {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify({ password }),
+      });
+
+      if (res.ok) {
+        onSuccess();
+      } else {
+        const data = await res.json();
+        setError(data.error || 'Login failed');
+      }
+    } catch {
+      setError('Connection error');
+    } finally {
+      setLoading(false);
+    }
+  }
+
+  return (
+    <div className="admin-login">
+      <form className="admin-login-card" onSubmit={handleSubmit}>
+        <div className="admin-login-header">
+          <h1>Immich Folio</h1>
+          <p>Admin Panel</p>
+        </div>
+
+        <div className="admin-field">
+          <label htmlFor="admin-password">Password</label>
+          <input
+            id="admin-password"
+            type="password"
+            value={password}
+            onChange={(e) => setPassword(e.target.value)}
+            placeholder="Enter admin password"
+            autoFocus
+            disabled={loading}
+          />
+        </div>
+
+        {error && <div className="admin-error">{error}</div>}
+
+        <button type="submit" className="admin-btn admin-btn-primary" disabled={loading}>
+          {loading ? 'Signing in...' : 'Sign In'}
+        </button>
+      </form>
+    </div>
+  );
+}