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/WorktreesOverviewModalView.axaml

@@ -9,7 +9,7 @@
         CanResize="True"
         WindowStartupLocation="CenterOwner"
         Background="{DynamicResource SurfaceBrush}"
-        SystemDecorations="BorderOnly"
+        WindowDecorations="BorderOnly"
         ExtendClientAreaToDecorationsHint="True"
         ExtendClientAreaTitleBarHeightHint="-1">