Migrate codebase to Avalonia 12 APIs

Apply Avalonia 12 migration changes: replace SystemDecorations with WindowDecorations and remove ExtendClientAreaChromeHints/ExtendClientAreaTitleBarHeightHint usages; update BindingPlugins removal logic (no-op); switch clipboard usage to ClipboardExtensions.SetTextAsync; update Bitmap.CopyPixels calls to the new signature. Replace TextBox.Watermark with PlaceholderText, convert NumberBox styles to FANumberBox and adjust templates, change Checked/Unchecked handlers to IsCheckedChanged, and adapt FluentIcons usages (SymbolIconSource -> FASymbol/FAFont/FluentIcon equivalents). Fix MainWindow partial classes to inherit Window and correct missing variables/fields/usings. Add migration docs/specs/tasks under .trae and include a small TestFluentIcons project for icon testing.

.trae/specs/avalonia-12-migration/checklist.md

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+# Checklist
+
+- [ ] `SettingsWindow.ApplyChromeMode()` 不再使用 `ExtendClientAreaChromeHints` 和 `SystemDecorations`
+- [ ] `ComponentEditorWindow.ApplyChromeMode()` 不再使用 `ExtendClientAreaChromeHints` 和 `SystemDecorations`
+- [ ] 所有 `.axaml` 文件中的 `SystemDecorations` 已替换为 `WindowDecorations`
+- [ ] `MainWindow.ComponentSystem.cs` 中 `centerLeft` 和 `positions` 变量已正确定义
+- [ ] `MainWindow.DesktopPaging.cs` 中 `child` 和 `_isThreeFingerOrRightDragSwipeActive` 变量已正确定义
+- [ ] `App.axaml.cs` 中 `BindingPlugins.DataValidators` 代码已移除
+- [ ] `DesktopComponentFailureView.cs` 使用 `ClipboardExtensions.SetTextAsync`
+- [ ] `MonetColorService.cs` 使用正确的 `Bitmap.CopyPixels` 签名
+- [ ] `SettingsWindow.axaml.cs` 使用 `FluentIcons.Avalonia.FluentIcon` 替代 `SymbolIconSource`
+- [ ] 所有 `TextBox.Watermark` 已替换为 `PlaceholderText`
+- [ ] `dotnet build LanMountainDesktop.slnx -c Debug` 0 errors, 0 warnings（过时 API 警告）
+- [ ] `dotnet test LanMountainDesktop.slnx -c Debug` 全部通过