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/models/Course.js

@@ -12,6 +12,19 @@ class Course {
         const rows = await db.query('SELECT * FROM courses WHERE id = ?', [id]);
         return rows[0];
     }
+
+    static async findByStudentId(studentId) {
+        const sql = `
+            SELECT c.*, u.name as teacher_name
+            FROM courses c
+            JOIN classes cl ON c.class_id = cl.id
+            JOIN students s ON cl.class_name = s.class
+            JOIN users u ON c.teacher_id = u.id
+            WHERE s.id = ?
+            ORDER BY c.course_code
+        `;
+        return await db.query(sql, [studentId]);
+    }
 }
 
 module.exports = Course;