refactor: 新建 Platform 平台差异层，主工程 P/Invoke 全部迁出

- 新增 Platform.Abstractions（接口+NoOp+PlatformLog 日志桥）、
  Platform.Windows、Platform.MacOS、Platform.Android 四个项目
- 迁移电源管理、原生对话框、桌面层嵌入、窗口置底/区域穿透、
  DWM 互操作、图标服务、包标识查询等全部 Windows P/Invoke 实现
- 主工程保留静态工厂门面，调用点不变；DllImport 扫描为零
- 更新 WindowPassthroughServiceTests 指向新位置（20/20 通过）

LanMountainDesktop.Platform.Abstractions/IPowerManagementService.cs

@@ -0,0 +1,48 @@
+namespace LanMountainDesktop.Platform.Abstractions;
+
+/// <summary>
+/// 电源管理服务接口。桌面平台提供关机/重启/注销/锁定/睡眠能力；
+/// 移动平台不提供此能力（使用 <see cref="NullPowerManagementService"/>）。
+/// </summary>
+public interface IPowerManagementService
+{
+    bool IsShutdownSupported { get; }
+    bool IsRestartSupported { get; }
+    bool IsLogoutSupported { get; }
+    bool IsLockSupported { get; }
+    bool IsSleepSupported { get; }
+
+    Task ShutdownAsync();
+    Task RestartAsync();
+    Task LogoutAsync();
+    Task LockAsync();
+    Task SleepAsync();
+
+    void ShowNativePowerUI(PowerAction action);
+}
+
+public enum PowerAction
+{
+    Shutdown,
+    Restart
+}
+
+/// <summary>
+/// 无操作实现。用于不支持电源管理的平台（如移动端）。
+/// </summary>
+public sealed class NullPowerManagementService : IPowerManagementService
+{
+    public bool IsShutdownSupported => false;
+    public bool IsRestartSupported => false;
+    public bool IsLogoutSupported => false;
+    public bool IsLockSupported => false;
+    public bool IsSleepSupported => false;
+
+    public Task ShutdownAsync() => Task.CompletedTask;
+    public Task RestartAsync() => Task.CompletedTask;
+    public Task LogoutAsync() => Task.CompletedTask;
+    public Task LockAsync() => Task.CompletedTask;
+    public Task SleepAsync() => Task.CompletedTask;
+
+    public void ShowNativePowerUI(PowerAction action) { }
+}