feat(app): 实现认证模块 — F2 Auth BLoC + 登录/注册/角色选择/班级码加入

新增文件 (10):
- data/models/user.dart — 用户+角色模型 (匹配后端 UserResp/RoleResp)
- data/models/auth_token.dart — 认证令牌模型 (匹配后端 LoginResp)
- data/repositories/auth_repository.dart — 认证仓库 (JWT 安全持久化 + PIPL 合规)
- features/auth/bloc/auth_bloc.dart — 认证 BLoC (8 种事件, 6 种状态)
- features/auth/bloc/auth_event.dart — 认证事件 (sealed class 穷尽匹配)
- features/auth/bloc/auth_state.dart — 认证状态 (Authenticated 含角色/班级码流程)
- features/auth/views/login_page.dart — 登录/注册页面 (重写占位页面)
- features/auth/views/role_selection_page.dart — 角色选择页 (4 种角色卡片)
- features/auth/views/class_code_join_page.dart — 班级码加入页 (6 位输入)

修改文件 (5):
- pubspec.yaml — 添加 flutter_secure_storage 依赖
- app.dart — 注入 AuthBloc + RepositoryProvider
- main.dart — 简化入口 (认证恢复在 BLoC 中处理)
- core/routing/app_router.dart — 添加认证路由守卫 + 2 新路由
- erp-diary/service/class_service.rs — 移除未使用的 PaginatorTrait import

验证: flutter analyze (0 error) + cargo check 通过

app/pubspec.lock

@@ -366,6 +366,54 @@ packages:
       url: "https://pub.flutter-io.cn"
     source: hosted
     version: "6.0.0"
+  flutter_secure_storage:
+    dependency: "direct main"
+    description:
+      name: flutter_secure_storage
+      sha256: "9cad52d75ebc511adfae3d447d5d13da15a55a92c9410e50f67335b6d21d16ea"
+      url: "https://pub.flutter-io.cn"
+    source: hosted
+    version: "9.2.4"
+  flutter_secure_storage_linux:
+    dependency: transitive
+    description:
+      name: flutter_secure_storage_linux
+      sha256: be76c1d24a97d0b98f8b54bce6b481a380a6590df992d0098f868ad54dc8f688
+      url: "https://pub.flutter-io.cn"
+    source: hosted
+    version: "1.2.3"
+  flutter_secure_storage_macos:
+    dependency: transitive
+    description:
+      name: flutter_secure_storage_macos
+      sha256: "6c0a2795a2d1de26ae202a0d78527d163f4acbb11cde4c75c670f3a0fc064247"
+      url: "https://pub.flutter-io.cn"
+    source: hosted
+    version: "3.1.3"
+  flutter_secure_storage_platform_interface:
+    dependency: transitive
+    description:
+      name: flutter_secure_storage_platform_interface
+      sha256: cf91ad32ce5adef6fba4d736a542baca9daf3beac4db2d04be350b87f69ac4a8
+      url: "https://pub.flutter-io.cn"
+    source: hosted
+    version: "1.1.2"
+  flutter_secure_storage_web:
+    dependency: transitive
+    description:
+      name: flutter_secure_storage_web
+      sha256: f4ebff989b4f07b2656fb16b47852c0aab9fed9b4ec1c70103368337bc1886a9
+      url: "https://pub.flutter-io.cn"
+    source: hosted
+    version: "1.2.1"
+  flutter_secure_storage_windows:
+    dependency: transitive
+    description:
+      name: flutter_secure_storage_windows
+      sha256: b20b07cb5ed4ed74fc567b78a72936203f587eba460af1df11281c9326cd3709
+      url: "https://pub.flutter-io.cn"
+    source: hosted
+    version: "3.1.2"
   flutter_test:
     dependency: "direct dev"
     description: flutter