feat(class-account): 学生端主体改为班级（班级码 + 班级密码）+ 数据按班级隔离

服务端:
- Class 新增 code(唯一)/passwordHash 与迁移(老数据回填 C00001 形式, 未设密码禁止登录)
- POST /auth/class-login: 班级码 + 班级密码 -> classSession 会话(sub=classId)
- PATCH /classes/:id/class-account: 管理员改班级码/重置密码(唯一性、格式校验)
- auth 中间件/socket 握手支持班级会话(sub 为 classId, 回查 Class 表, 加入 user:{classId} 房间)
- lib/session.ts: 班级设备代全班操作(通知已读/作业完成/成绩总览按全班读写), 普通学生账号行为不变
- 叫人消息同时广播到 user:{classId}, 班级设备立即上岛
- 班级列表/详情按权限返回班级码与密码状态(哈希不外泄)

客户端:
- 登录页改为班级码 + 班级密码(个人学生入口移除, /auth/login 接口保留兼容)
- 作业按钮/成绩标题按班级会话显示(全班标记完成 / 本班成绩)
- 冒烟脚本自动准备班级账号凭据(兼容全新与升级安装)

Web 管理端:
- 班级管理新增「班级账号」列与设置弹窗(改班级码/重置密码/随机生成)
- 新建班级可选自定义班级码, 默认由服务端生成

验证: verify:e2e 125/125(新增班级账号 18 项) verify:web 18/18 verify:desktop 40/40
      (login=高一(1)班 classSession=true)
文档: README 新增「学生端班级账号」章节、API/迁移/验收/默认账号更新; 安装包 README 同步

packages/server/prisma/seed.ts

@@ -84,10 +84,12 @@ async function main(): Promise<void> {
   const random = createRandom(20260901);
 
   // 同一角色共用密码，只需各哈希一次
-  const [adminHash, teacherHash, studentHash] = await Promise.all([
+  const [adminHash, teacherHash, studentHash, classPasswordHash] = await Promise.all([
     hashPassword('admin123'),
     hashPassword('teacher123'),
     hashPassword('student123'),
+    // 班级账号默认密码（学生端「班级登录」）
+    hashPassword('123456'),
   ]);
 
   const admin = await prisma.user.create({
@@ -101,16 +103,23 @@ async function main(): Promise<void> {
     data: { username: 'teacher2', name: '李老师', role: 'TEACHER', passwordHash: teacherHash },
   });
 
+  // 班级账号：班级码（学生端「班级登录」的账号）+ 班级密码（默认 123456，可在班级管理里重置）
   const classSeeds = [
-    { name: '高一(1)班', grade: '高一', teacherId: teacher1.id },
-    { name: '高一(2)班', grade: '高一', teacherId: teacher2.id },
-    { name: '高二(3)班', grade: '高二', teacherId: teacher1.id },
+    { name: '高一(1)班', grade: '高一', teacherId: teacher1.id, code: 'G101' },
+    { name: '高一(2)班', grade: '高一', teacherId: teacher2.id, code: 'G102' },
+    { name: '高二(3)班', grade: '高二', teacherId: teacher1.id, code: 'G203' },
   ];
 
   const classes = [];
   for (const [index, seed] of classSeeds.entries()) {
     const created = await prisma.class.create({
-      data: { name: seed.name, grade: seed.grade, teacherId: seed.teacherId },
+      data: {
+        name: seed.name,
+        grade: seed.grade,
+        teacherId: seed.teacherId,
+        code: seed.code,
+        passwordHash: classPasswordHash,
+      },
     });
     classes.push({ ...created, room: CLASS_ROOMS[index] ?? '教学楼 A101' });
   }