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/Modals/WorktreeModalView.axaml

@@ -8,7 +8,7 @@
         Width="1100" Height="720"
         MinWidth="640" MinHeight="400"
         WindowStartupLocation="CenterOwner"
-        SystemDecorations="BorderOnly"
+        WindowDecorations="BorderOnly"
         ExtendClientAreaToDecorationsHint="True"
         ExtendClientAreaTitleBarHeightHint="-1"
         Background="Transparent"