feat(frontend): add a dedicated 403 forbidden page

Show /403 instead of toasting or staying on the denied screen when
the API returns 403 or a non-admin opens an admin route.

frontend/app/403/page.tsx

@@ -0,0 +1,7 @@
+'use client';
+
+import { ForbiddenPage } from '@/components/layout/forbidden-page';
+
+export default function ForbiddenRoutePage() {
+  return <ForbiddenPage />;
+}

frontend/app/forbidden.tsx

@@ -0,0 +1,8 @@
+'use client';
+
+import { ForbiddenPage } from '@/components/layout/forbidden-page';
+
+/** Next.js convention: rendered when forbidden() is called. */
+export default function Forbidden() {
+  return <ForbiddenPage />;
+}