feat(学生): 添加学生课程查询功能

- 后端添加获取学生课程列表的API路由
- 实现Course模型中的findByStudentId方法查询学生课程
- 新增学生控制器的getCourses方法处理课程请求
- 前端添加课程表格展示及刷新功能

backend/routes/student.js

@@ -5,5 +5,6 @@ const { requireAuth, requireRole } = require('../middleware/auth');
 
 router.get('/grades', requireAuth, requireRole(['student']), StudentController.getGrades);
 router.get('/grades/:id', requireAuth, requireRole(['student']), StudentController.getGradeDetails);
+router.get('/courses', requireAuth, requireRole(['student']), StudentController.getCourses);
 
 module.exports = router;