feat(app): pnpm 一键启动 + Flutter Web 编译修复

1. 新增 pnpm start:dev / pnpm start:stop 命令
   - scripts/dev.mjs: 跨平台启动脚本（后端+管理端+学生端）
   - scripts/stop.mjs: 端口清理停止脚本
   - 根 package.json 定义 pnpm 脚本

2. 修复 Flutter Web 编译（Isar 3.x + flutter_secure_storage 不兼容）
   - isar_database: 条件导出，Web 用空 stub
   - isar_journal_repository: 条件导出，Web 用空 stub
   - sync_engine: 条件导出，Web 用内存队列（无 Isar 持久化）
   - 移除 flutter_secure_storage（v9 web 插件用 dart:html）
   - 新增 SecureTokenStore 接口 + shared_preferences 实现
   - auth_repository 改用 SecureTokenStore 接口

app/pubspec.lock

@@ -406,54 +406,6 @@ packages:
       url: "https://pub.flutter-io.cn"
     source: hosted
     version: "2.0.34"
-  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