feat: P2-12 Teacher profile/workspace + P2-13 Multi-class comparison

P2-12 Teacher Management:
- Add profile JSON field (subjects, joinedAt, qualifications) to User entity
- Add GET/PUT /rbac/users/:id/profile endpoints with UpdateProfileDto
- Add GET /rbac/teacher-workspace endpoint returning assignedClasses,
  todaySchedules, and myStudents
- Create /teacher-workspace page with tabs (My Classes, Today's Schedule,
  My Students)
- Add route with PermissionRoute and menu item in MainLayout

P2-13 Student Archive Multi-class:
- Add GET /students/:id/compare-classes endpoint returning side-by-side
  enrollment data with per-class attendance statistics
- Students module now includes ClassStudent and AttendanceRecord entities
- No 2-enrollment cap existed in the codebase; student enrollments
  already return all records via class-student table

apps/admin/src/layouts/MainLayout.tsx

@@ -23,6 +23,7 @@ import {
   SafetyOutlined,
   KeyOutlined,
   CheckCircleOutlined,
+  LaptopOutlined,
 } from '@ant-design/icons';
 import { usePermission } from '../hooks/usePermission';
 
@@ -51,6 +52,7 @@ const allMenuItems: MenuItemType[] = [
   { key: '/deposits', icon: <WalletOutlined />, label: '押金管理', permission: 'deposit:view' },
   { key: '/bills', icon: <FileTextOutlined />, label: '账单管理', permission: 'bill:view' },
   { key: '/classes', icon: <TeamOutlined />, label: '班级管理', permission: 'class:view' },
+  { key: '/teacher-workspace', icon: <LaptopOutlined />, label: '教师工作台', permission: 'class:view' },
   {
     key: 'classroom-group',
     icon: <ReadOutlined />,