feat(student): 完成 student 模块 v4 剩余修复

- P1-4.2: 新增班级详情页 courses/[classId]，展示教师/学校/教室信息与课表

- P2-2.5: 今日课表卡片高亮当前/下一节课（useMemo 实时计算）

- P2-3.9: 作业作答进度网格支持点击跳转题目（scrollIntoView）

- P2-3.10: 作业复习视图显示正确答案（选择/判断/文本题）

- P2-4.4: 课程列表支持按班级名/教师/学校搜索

- P2-5.2: 成绩页新增趋势折线图组件 GradeTrendCard

- P2-9.2/9.3: 诊断报告新增历史记录卡片与弱点练习入口

- P2-10.2: 选课列表支持搜索与选课模式筛选

- P2-11.3: 修复教材阅读页全屏溢出

- P3-1.5: 面包屑保留首个角色段作为根上下文

- P3-7.3: 课表项支持点击跳转至班级详情页（ScheduleList href）

src/modules/classes/data-access-students.ts

@@ -192,6 +192,48 @@ export const getStudentClasses = cache(async (studentId: string): Promise<Studen
   return list
 })
 
+/**
+ * Get a single class details for a student (verifies enrollment).
+ * Returns null if the student is not enrolled in the class.
+ */
+export const getStudentClassById = cache(
+  async (studentId: string, classId: string): Promise<StudentEnrolledClass | null> => {
+    const sid = studentId.trim()
+    const cid = classId.trim()
+    if (!sid || !cid) return null
+
+    const rows = await db
+      .select({
+        id: classes.id,
+        schoolName: classes.schoolName,
+        name: classes.name,
+        grade: classes.grade,
+        homeroom: classes.homeroom,
+        room: classes.room,
+        teacherName: users.name,
+        teacherEmail: users.email,
+      })
+      .from(classEnrollments)
+      .innerJoin(classes, eq(classes.id, classEnrollments.classId))
+      .leftJoin(users, eq(users.id, classes.teacherId))
+      .where(and(eq(classEnrollments.studentId, sid), eq(classEnrollments.classId, cid), eq(classEnrollments.status, "active")))
+      .limit(1)
+
+    if (rows.length === 0) return null
+    const r = rows[0]
+    return {
+      id: r.id,
+      schoolName: r.schoolName,
+      name: r.name,
+      grade: r.grade,
+      homeroom: r.homeroom,
+      room: r.room,
+      teacherName: r.teacherName,
+      teacherEmail: r.teacherEmail,
+    }
+  }
+)
+
 export const getClassStudents = cache(
   async (params?: { classId?: string; q?: string; status?: string; teacherId?: string }): Promise<ClassStudent[]> => {
     const teacherId = params?.teacherId ?? (await getSessionTeacherId())