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/IMainWindowDesktopLayerService.cs

@@ -0,0 +1,30 @@
+using Avalonia.Controls;
+
+namespace LanMountainDesktop.Platform.Abstractions;
+
+/// <summary>
+/// 主窗口桌面层服务接口。将主窗口嵌入系统桌面图标层（仅 Windows 支持）。
+/// </summary>
+public interface IMainWindowDesktopLayerService
+{
+    bool IsSupported { get; }
+    void EnableOrRefresh(Window window);
+    void Disable(Window window);
+}
+
+/// <summary>
+/// 无操作实现。用于不支持桌面层嵌入的平台（Linux/macOS/移动端）。
+/// </summary>
+public sealed class NullMainWindowDesktopLayerService : IMainWindowDesktopLayerService
+{
+    public bool IsSupported => false;
+
+    public void EnableOrRefresh(Window window)
+    {
+        PlatformLog.Info("MainWindowDesktopLayer", "Desktop layer requested on an unsupported platform.");
+    }
+
+    public void Disable(Window window)
+    {
+    }
+}

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) { }
+}

LanMountainDesktop.Platform.Abstractions/IWindowPassthroughServices.cs

@@ -0,0 +1,68 @@
+using Avalonia;
+using Avalonia.Controls;
+
+namespace LanMountainDesktop.Platform.Abstractions;
+
+/// <summary>
+/// 窗口置底服务接口。使组件窗口保持在桌面层（Z 序最底）。
+/// </summary>
+public interface IWindowBottomMostService
+{
+    void SetupBottomMost(Window window);
+    void SendToBottom(Window window);
+    PixelPoint GetScreenPosition(Window window);
+    bool SetScreenPosition(Window window, PixelPoint position, bool queueOnFailure = false);
+    bool IsBottomMostSupported { get; }
+}
+
+/// <summary>
+/// 窗口交互区域定义。区域外的点击穿透到桌面。
+/// </summary>
+public readonly record struct WindowInteractiveRegion(
+    Rect Bounds,
+    double CornerRadius,
+    Matrix? ClientToRegionTransform = null,
+    Rect? ClientClipBounds = null,
+    double ClientClipCornerRadius = 0d);
+
+/// <summary>
+/// 区域点击穿透服务接口。
+/// </summary>
+public interface IRegionPassthroughService
+{
+    void SetInteractiveRegions(Window window, IReadOnlyList<WindowInteractiveRegion> interactiveRegions);
+    void ClearInteractiveRegions(Window window);
+    bool IsRegionPassthroughSupported { get; }
+}
+
+/// <summary>
+/// 无操作实现：非 Windows 平台窗口置底不可用。
+/// </summary>
+public sealed class NullWindowBottomMostService : IWindowBottomMostService
+{
+    public bool IsBottomMostSupported => false;
+
+    public void SetupBottomMost(Window window) { }
+
+    public void SendToBottom(Window window) { }
+
+    public PixelPoint GetScreenPosition(Window window) => window.Position;
+
+    public bool SetScreenPosition(Window window, PixelPoint position, bool queueOnFailure = false)
+    {
+        window.Position = position;
+        return true;
+    }
+}
+
+/// <summary>
+/// 无操作实现：非 Windows 平台区域穿透不可用。
+/// </summary>
+public sealed class NullRegionPassthroughService : IRegionPassthroughService
+{
+    public bool IsRegionPassthroughSupported => false;
+
+    public void SetInteractiveRegions(Window window, IReadOnlyList<WindowInteractiveRegion> interactiveRegions) { }
+
+    public void ClearInteractiveRegions(Window window) { }
+}

LanMountainDesktop.Platform.Abstractions/LanMountainDesktop.Platform.Abstractions.csproj

@@ -0,0 +1,11 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Avalonia" />
+  </ItemGroup>
+</Project>

LanMountainDesktop.Platform.Abstractions/PlatformLog.cs

@@ -0,0 +1,35 @@
+namespace LanMountainDesktop.Platform.Abstractions;
+
+/// <summary>
+/// 平台层日志桥。平台实现项目不引用宿主，
+/// 宿主在启动时通过 <see cref="SetSink"/> 接入自己的日志系统（AppLogger）。
+/// 未接入时日志静默丢弃。
+/// </summary>
+public static class PlatformLog
+{
+    private static IPlatformLogSink? _sink;
+
+    public static void SetSink(IPlatformLogSink sink)
+    {
+        ArgumentNullException.ThrowIfNull(sink);
+        _sink = sink;
+    }
+
+    public static void Info(string category, string message) => _sink?.Info(category, message);
+
+    public static void Warn(string category, string message, Exception? exception = null) =>
+        _sink?.Warn(category, message, exception);
+
+    public static void Error(string category, string message, Exception? exception = null) =>
+        _sink?.Error(category, message, exception);
+}
+
+/// <summary>
+/// 平台层日志输出目标。
+/// </summary>
+public interface IPlatformLogSink
+{
+    void Info(string category, string message);
+    void Warn(string category, string message, Exception? exception = null);
+    void Error(string category, string message, Exception? exception = null);
+}