refactor(classes): preserve student membership history

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

@@ -19,6 +19,7 @@ interface ClassStudent {
   studentName: string;
   studentNo: string;
   joinDate: string;
+  leaveDate: string | null;
   status: string;
 }
 
@@ -305,6 +306,7 @@ const ClassDetailPage: React.FC = () => {
     { title: '姓名', dataIndex: 'studentName' },
     { title: '学号', dataIndex: 'studentNo' },
     { title: '加入日期', dataIndex: 'joinDate' },
+    { title: '离班日期', dataIndex: 'leaveDate', render: (v: string | null) => v || '-' },
     {
       title: '状态',
       dataIndex: 'status',
@@ -316,11 +318,12 @@ const ClassDetailPage: React.FC = () => {
     },
     {
       title: '操作',
-      render: (_: unknown, r: ClassStudent) => (
-        <Popconfirm title="确认移除？" onConfirm={() => handleRemoveStudent(r.studentId)}>
-          <PermissionButton permission="class:edit" size="small" danger>移除</PermissionButton>
-        </Popconfirm>
-      ),
+      render: (_: unknown, r: ClassStudent) =>
+        r.status === 'active' ? (
+          <Popconfirm title="确认移除？" onConfirm={() => handleRemoveStudent(r.studentId)}>
+            <PermissionButton permission="class:edit" size="small" danger>移除</PermissionButton>
+          </Popconfirm>
+        ) : null,
     },
   ];