Introduce Avalonia as a new cross-platform UI option, including new XAML and code-behind files for the application and main window. Update Program.cs to support a --avalonia launch argument and add corresponding launch profile. Integrate Avalonia NuGet packages and ensure DataModel supports UI-thread invocation for all frontends. All business logic and state are shared via DI, enabling consistent behavior across WinForms, WPF, and Avalonia.
Refactored Program.cs to accept command-line arguments and select between WinForms (default) and WPF UI based on the presence of --wpf. Removed old commented-out UI selection code. Updated launchSettings.json to provide separate profiles for WinForms and WPF launches. If WPF is requested but unavailable, the app falls back to WinForms.
