fix+build(client/windows): 复查修缮——一键构建脚本 + 修 README dart-define 漏项 + 清 lint

- 新增 client/windows/build.ps1：拉内核→带 --dart-define 构建→出安装包一键串联，
  规避 Release 漏 --dart-define 导致默认连 localhost、登录「网络请求失败」的坑。
- README：Release 命令补回 --dart-define 并加显式警告，指向一键脚本。
- 清 4 个 lint warning：desktop_vpn_bridge 多余 cast→pattern match、去多余非空断言；
  nav_sidebar 删未用 import；vpn_bridge_mock 加 killSwitchEnabled getter。
- 同步 window_manager/tray_manager/screen_retriever 插件注册生成文件(win+macos)。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

client/windows/flutter/generated_plugin_registrant.cc

@@ -7,8 +7,17 @@
 #include "generated_plugin_registrant.h"
 
 #include <flutter_secure_storage_windows/flutter_secure_storage_windows_plugin.h>
+#include <screen_retriever_windows/screen_retriever_windows_plugin_c_api.h>
+#include <tray_manager/tray_manager_plugin.h>
+#include <window_manager/window_manager_plugin.h>
 
 void RegisterPlugins(flutter::PluginRegistry* registry) {
   FlutterSecureStorageWindowsPluginRegisterWithRegistrar(
       registry->GetRegistrarForPlugin("FlutterSecureStorageWindowsPlugin"));
+  ScreenRetrieverWindowsPluginCApiRegisterWithRegistrar(
+      registry->GetRegistrarForPlugin("ScreenRetrieverWindowsPluginCApi"));
+  TrayManagerPluginRegisterWithRegistrar(
+      registry->GetRegistrarForPlugin("TrayManagerPlugin"));
+  WindowManagerPluginRegisterWithRegistrar(
+      registry->GetRegistrarForPlugin("WindowManagerPlugin"));
 }