chore(ui): clear build warnings

- Guard Windows-only ServiceController/registry calls behind SupportedOSPlatform
  and OperatingSystem.IsWindows() (CA1416)
- Initialize test-only ctor fields with null! (CS8618)
- Migrate obsolete Avalonia APIs: Watermark -> PlaceholderText,
  SystemDecorations -> WindowDecorations

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ClaudeDo.Ui/Views/MainWindow.axaml

@@ -10,7 +10,7 @@
         Background="{DynamicResource VoidBrush}"
         Icon="avares://ClaudeDo.Ui/Assets/ClaudeTask.ico"
         CanResize="True"
-        SystemDecorations="BorderOnly"
+        WindowDecorations="BorderOnly"
         ExtendClientAreaToDecorationsHint="True"
         ExtendClientAreaTitleBarHeightHint="-1">
   <Window.Resources>