feat(mvp): phase 6 - admin panel

Add admin layout with auth guard, user management (CRUD + group membership),
group management, system settings (auth mode, registration, theme, healthcheck),
permission editor component, and global search API endpoint.

src/routes/admin/+layout.server.ts

@@ -0,0 +1,8 @@
+import type { LayoutServerLoad } from './$types.js';
+import { requireAdmin } from '$lib/server/middleware/authorize.js';
+
+export const load: LayoutServerLoad = async (event) => {
+	const user = requireAdmin(event);
+
+	return { user };
+};