fix(admin): add layout guard and redirect /admin to /admin/scrape

- Add +layout.server.ts to enforce admin role check at layout level,
  preventing 404 on /admin and protecting all sub-routes centrally
- Add +page.server.ts to redirect /admin → /admin/scrape (was 404)

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

@@ -0,0 +1,8 @@
+import { redirect } from '@sveltejs/kit';
+import type { LayoutServerLoad } from './$types';
+
+export const load: LayoutServerLoad = async ({ locals }) => {
+	if (locals.user?.role !== 'admin') {
+		redirect(302, '/');
+	}
+};

ui/src/routes/admin/+page.server.ts

@@ -0,0 +1,6 @@
+import { redirect } from '@sveltejs/kit';
+import type { PageServerLoad } from './$types';
+
+export const load: PageServerLoad = async () => {
+	redirect(302, '/admin/scrape');
+};