feat: add Classes list page with CRUD modal

apps/admin/src/pages/Classes/detail.tsx

@@ -0,0 +1,9 @@
+import React from 'react';
+import { Card } from 'antd';
+
+// Placeholder — Task 1.5 will implement the full detail page.
+const ClassDetailPage: React.FC = () => {
+  return <Card>班级详情 — 待实现</Card>;
+};
+
+export default ClassDetailPage;