Add dialog service abstraction for platform-specific dialogs:
- IDialogService interface with folder picker, message, confirmation,
diff preview, and validation results methods
- AvaloniaDialogService implementation using MsBox.Avalonia
- Basic implementations for ShowDiffPreviewAsync and ShowValidationResultsAsync
(full dialogs to be implemented in Tasks 22-23)
- Add MessageBox.Avalonia package reference
Joseph Doherty
7dd4c46cb7