feat(avalonia): wire up file menu commands in MainWindow

- Add menu bar with File menu (Open, Save, Save As, Recent Files, Examples, Preferences, About)
- Add keyboard shortcuts for all file operations
- Implement file operation commands in MainWindowViewModel
- Add IFileService dependency to MainWindowViewModel
- Implement OpenFileCommand, SaveFileCommand, SaveAsCommand
- Implement OpenExampleCommand, ShowPreferencesCommand, ShowAboutCommand
- Add SetParentWindow method for showing dialogs
- Wire up command methods to show dialogs and handle results
- Update all MainWindowViewModel tests to include mock FileService
- All 223 tests passing

Author: lookbusy1344