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/windows/flutter/generated_plugin_registrant.cc

@@ -8,7 +8,6 @@
 
 #include <connectivity_plus/connectivity_plus_windows_plugin.h>
 #include <file_selector_windows/file_selector_windows.h>
-#include <flutter_secure_storage_windows/flutter_secure_storage_windows_plugin.h>
 #include <isar_flutter_libs/isar_flutter_libs_plugin.h>
 #include <permission_handler_windows/permission_handler_windows_plugin.h>
 #include <share_plus/share_plus_windows_plugin_c_api.h>
@@ -19,8 +18,6 @@ void RegisterPlugins(flutter::PluginRegistry* registry) {
       registry->GetRegistrarForPlugin("ConnectivityPlusWindowsPlugin"));
   FileSelectorWindowsRegisterWithRegistrar(
       registry->GetRegistrarForPlugin("FileSelectorWindows"));
-  FlutterSecureStorageWindowsPluginRegisterWithRegistrar(
-      registry->GetRegistrarForPlugin("FlutterSecureStorageWindowsPlugin"));
   IsarFlutterLibsPluginRegisterWithRegistrar(
       registry->GetRegistrarForPlugin("IsarFlutterLibsPlugin"));
   PermissionHandlerWindowsPluginRegisterWithRegistrar(