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/Services/InstallerLocator.cs

@@ -7,7 +7,8 @@ public sealed class InstallerLocator
     private const string UninstallerSubdir = "uninstaller";
 
     public string? Find()
-        => FindByWalkingUp(AppContext.BaseDirectory) ?? FindByRegistry();
+        => FindByWalkingUp(AppContext.BaseDirectory)
+           ?? (OperatingSystem.IsWindows() ? FindByRegistry() : null);
 
     public string? FindByWalkingUp(string startDir)
     {