Add Avalonia UI frontend alongside WinForms and WPF

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.

imagecatalog/AvaloniaApp.axaml.cs

@@ -0,0 +1,20 @@
+using Avalonia.Controls.ApplicationLifetimes;
+using Avalonia.Markup.Xaml;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace ImageCatalog_2;
+
+public partial class AvaloniaApp : Avalonia.Application
+{
+    public override void Initialize() => AvaloniaXamlLoader.Load(this);
+
+    public override void OnFrameworkInitializationCompleted()
+    {
+        if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
+        {
+            var model = Program.ServiceProvider.GetRequiredService<DataModel>();
+            desktop.MainWindow = new AvaloniaMainWindow(model);
+        }
+        base.OnFrameworkInitializationCompleted();
+    }
+}