feat: 引入 CloudSync 核心能力并新增 Avalonia 桌面端与发布脚本

- 后端：新增 CloudSync 认证/权限/端点/服务与 DTO
- 数据：新增用户/会话/安全策略实体与 EF Core migrations
- 前端：新增云同步设置 UI、客户端与本地存储；Vite 支持 maui 构建输出到 wwwroot
- 桌面端：新增 Avalonia 项目、内置 WebServer、托盘与 Windows 全局热键
- 发布/构建：新增 Windows/Linux 发布脚本与统一入口；调整 MAUI 资源与安装包配置
- 文档：同步更新 README/docs 与协作规则

src/Hua.Todo.Avalonia/Services/Platforms/WindowsKeyboardHandler.cs

@@ -0,0 +1,138 @@
+using System;
+using System.Runtime.InteropServices;
+
+namespace Hua.Todo.Avalonia.Services.Platforms;
+
+/// <summary>
+/// Windows 全局键盘事件处理器。
+/// 用于监听按键并向上层暴露事件（例如 Esc），以保证在 WebView 获得焦点时依旧可触发窗口级交互。
+/// </summary>
+public sealed class WindowsKeyboardHandler : IDisposable
+{
+    private readonly KeyboardHook _keyboardHook;
+    private bool _isDisposed;
+
+    /// <summary>
+    /// 当检测到 Esc 键抬起时触发。
+    /// </summary>
+    public event EventHandler? EscKeyPressed;
+
+    /// <summary>
+    /// 创建 <see cref="WindowsKeyboardHandler"/>。
+    /// </summary>
+    public WindowsKeyboardHandler()
+    {
+        _keyboardHook = new KeyboardHook();
+        _keyboardHook.KeyPressed += OnKeyPressed;
+    }
+
+    /// <summary>
+    /// 开始监听键盘事件。
+    /// </summary>
+    public void Start()
+    {
+        _keyboardHook.Hook();
+    }
+
+    private void OnKeyPressed(object? sender, KeyPressedEventArgs e)
+    {
+        if (e.Key == 0x1B && !e.IsKeyDown)
+        {
+            EscKeyPressed?.Invoke(this, EventArgs.Empty);
+        }
+    }
+
+    /// <inheritdoc />
+    public void Dispose()
+    {
+        if (_isDisposed) return;
+        _isDisposed = true;
+
+        _keyboardHook.Unhook();
+        _keyboardHook.KeyPressed -= OnKeyPressed;
+    }
+
+    private sealed class KeyboardHook : IDisposable
+    {
+        private const int WhKeyboardLl = 13;
+        private const int WmKeyDown = 0x0100;
+        private const int WmKeyUp = 0x0101;
+        private const int WmSysKeyDown = 0x0104;
+        private const int WmSysKeyUp = 0x0105;
+
+        private readonly LowLevelKeyboardProc _proc;
+        private IntPtr _hookId = IntPtr.Zero;
+        private bool _isDisposed;
+
+        public event EventHandler<KeyPressedEventArgs>? KeyPressed;
+
+        public KeyboardHook()
+        {
+            _proc = HookCallback;
+        }
+
+        public void Hook()
+        {
+            if (_hookId != IntPtr.Zero) return;
+            _hookId = SetWindowsHookEx(WhKeyboardLl, _proc, GetModuleHandle(null), 0);
+        }
+
+        public void Unhook()
+        {
+            if (_hookId == IntPtr.Zero) return;
+            UnhookWindowsHookEx(_hookId);
+            _hookId = IntPtr.Zero;
+        }
+
+        private IntPtr HookCallback(int nCode, IntPtr wParam, IntPtr lParam)
+        {
+            if (nCode >= 0)
+            {
+                var vkCode = Marshal.ReadInt32(lParam);
+                var isKeyDown = wParam == (IntPtr)WmKeyDown || wParam == (IntPtr)WmSysKeyDown;
+                var isKeyUp = wParam == (IntPtr)WmKeyUp || wParam == (IntPtr)WmSysKeyUp;
+
+                if (isKeyDown || isKeyUp)
+                {
+                    KeyPressed?.Invoke(this, new KeyPressedEventArgs(vkCode, isKeyDown));
+                }
+            }
+
+            return CallNextHookEx(_hookId, nCode, wParam, lParam);
+        }
+
+        public void Dispose()
+        {
+            if (_isDisposed) return;
+            _isDisposed = true;
+            Unhook();
+        }
+
+        [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
+        private static extern IntPtr SetWindowsHookEx(int idHook, LowLevelKeyboardProc lpfn, IntPtr hMod, uint dwThreadId);
+
+        [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
+        private static extern bool UnhookWindowsHookEx(IntPtr hhk);
+
+        [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
+        private static extern IntPtr CallNextHookEx(IntPtr hhk, int nCode, IntPtr wParam, IntPtr lParam);
+
+        [DllImport("kernel32.dll", CharSet = CharSet.Auto, SetLastError = true)]
+        private static extern IntPtr GetModuleHandle(string? lpModuleName);
+    }
+
+    private delegate IntPtr LowLevelKeyboardProc(int nCode, IntPtr wParam, IntPtr lParam);
+
+    private sealed class KeyPressedEventArgs : EventArgs
+    {
+        public int Key { get; }
+        public bool IsKeyDown { get; }
+
+        public KeyPressedEventArgs(int key, bool isKeyDown)
+        {
+            Key = key;
+            IsKeyDown = isKeyDown;
+        }
+    }
+}
+