feat(client): 新增 Windows 桌面端（sing-box TUN 全局代理）

Windows 与 macOS 桌面同路线：Flutter 应用 + sing-box.exe 子进程 + wintun TUN
接管整机流量。Dart/桥接层此前已 Windows-ready（kernel_process._buildCommand /
_resolveBinaryPath、desktop_vpn_bridge 配置路径、fetch-desktop-bin.sh），本次补
原生外壳 + 提权 + 打包：

- flutter create --platforms=windows 生成 windows/ runner（flutter_secure_storage
  已注册 windows 插件，token 存储可用）。
- runner.exe.manifest 设 requestedExecutionLevel=requireAdministrator：启动 UAC
  提权，sing-box 子进程继承管理员权以创建 wintun TUN（与 _buildCommand 既有设计一致）。
- windows/CMakeLists.txt 增 install 规则：打包时把 app/kernel/dist/desktop/windows-*/
  {sing-box.exe,wintun.dll} 拷到 app exe 同级目录（存在才拷，缺失告警，类比 android
  build.gradle 占位）。
- kernel_process.dart 启动时校验 wintun.dll 与 sing-box.exe 同目录，缺失早警。
- windows/README.md：拉内核二进制 / 管理员终端 flutter run / 打包 / 出网验证步骤。

提权用 requireAdministrator（每次 UAC），正式版可换特权 helper（BACKLOG，类比 SMJobBless）。

本机（macOS）验证：flutter analyze 无 error。Windows 真机 build/连通验证待后续
（无 Windows 机器，状态同 Android/iOS：代码就绪、未真机验证）。

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

client/windows/runner/win32_window.h

@@ -0,0 +1,102 @@
+#ifndef RUNNER_WIN32_WINDOW_H_
+#define RUNNER_WIN32_WINDOW_H_
+
+#include <windows.h>
+
+#include <functional>
+#include <memory>
+#include <string>
+
+// A class abstraction for a high DPI-aware Win32 Window. Intended to be
+// inherited from by classes that wish to specialize with custom
+// rendering and input handling
+class Win32Window {
+ public:
+  struct Point {
+    unsigned int x;
+    unsigned int y;
+    Point(unsigned int x, unsigned int y) : x(x), y(y) {}
+  };
+
+  struct Size {
+    unsigned int width;
+    unsigned int height;
+    Size(unsigned int width, unsigned int height)
+        : width(width), height(height) {}
+  };
+
+  Win32Window();
+  virtual ~Win32Window();
+
+  // Creates a win32 window with |title| that is positioned and sized using
+  // |origin| and |size|. New windows are created on the default monitor. Window
+  // sizes are specified to the OS in physical pixels, hence to ensure a
+  // consistent size this function will scale the inputted width and height as
+  // as appropriate for the default monitor. The window is invisible until
+  // |Show| is called. Returns true if the window was created successfully.
+  bool Create(const std::wstring& title, const Point& origin, const Size& size);
+
+  // Show the current window. Returns true if the window was successfully shown.
+  bool Show();
+
+  // Release OS resources associated with window.
+  void Destroy();
+
+  // Inserts |content| into the window tree.
+  void SetChildContent(HWND content);
+
+  // Returns the backing Window handle to enable clients to set icon and other
+  // window properties. Returns nullptr if the window has been destroyed.
+  HWND GetHandle();
+
+  // If true, closing this window will quit the application.
+  void SetQuitOnClose(bool quit_on_close);
+
+  // Return a RECT representing the bounds of the current client area.
+  RECT GetClientArea();
+
+ protected:
+  // Processes and route salient window messages for mouse handling,
+  // size change and DPI. Delegates handling of these to member overloads that
+  // inheriting classes can handle.
+  virtual LRESULT MessageHandler(HWND window,
+                                 UINT const message,
+                                 WPARAM const wparam,
+                                 LPARAM const lparam) noexcept;
+
+  // Called when CreateAndShow is called, allowing subclass window-related
+  // setup. Subclasses should return false if setup fails.
+  virtual bool OnCreate();
+
+  // Called when Destroy is called.
+  virtual void OnDestroy();
+
+ private:
+  friend class WindowClassRegistrar;
+
+  // OS callback called by message pump. Handles the WM_NCCREATE message which
+  // is passed when the non-client area is being created and enables automatic
+  // non-client DPI scaling so that the non-client area automatically
+  // responds to changes in DPI. All other messages are handled by
+  // MessageHandler.
+  static LRESULT CALLBACK WndProc(HWND const window,
+                                  UINT const message,
+                                  WPARAM const wparam,
+                                  LPARAM const lparam) noexcept;
+
+  // Retrieves a class instance pointer for |window|
+  static Win32Window* GetThisFromHandle(HWND const window) noexcept;
+
+  // Update the window frame's theme to match the system theme.
+  static void UpdateTheme(HWND const window);
+
+  bool quit_on_close_ = false;
+
+  // window handle for top level window.
+  HWND window_handle_ = nullptr;
+
+  // window handle for hosted content.
+  HWND child_content_ = nullptr;
+};
+
+#endif  // RUNNER_WIN32_WINDOW_H_