feat: add useCampus hook and CampusSwitcher component

apps/admin/src/components/CampusSwitcher.tsx

@@ -0,0 +1,36 @@
+import React from 'react';
+import { Select, Typography } from 'antd';
+import { EnvironmentOutlined } from '@ant-design/icons';
+import { useCampus } from '../hooks/useCampus';
+
+const CampusSwitcher: React.FC = () => {
+  const { campuses, currentId, switchCampus, loading } = useCampus();
+
+  if (campuses.length <= 1) {
+    return (
+      <Typography.Text style={{ color: '#fff', marginRight: 24 }}>
+        <EnvironmentOutlined style={{ marginRight: 4 }} />
+        {campuses[0]?.name || '主校区'}
+      </Typography.Text>
+    );
+  }
+
+  const options = [
+    ...campuses.map((c) => ({ value: String(c.id), label: c.name })),
+    { value: '', label: '全部校区' },
+  ];
+
+  return (
+    <Select
+      value={currentId || undefined}
+      onChange={switchCampus}
+      options={options}
+      loading={loading}
+      style={{ minWidth: 140, marginRight: 24 }}
+      variant="borderless"
+      popupMatchSelectWidth={false}
+    />
+  );
+};
+
+export default CampusSwitcher;