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/DataModel.cs

@@ -207,12 +207,20 @@ namespace ImageCatalog_2
             return digits;
         }
 
+        /// <summary>
+        /// Optional UI-thread invoker set by the active UI layer (WPF, Avalonia, etc.).
+        /// When set, <see cref="InvokeOnUiThreadAsync"/> uses this delegate instead of the WPF dispatcher.
+        /// </summary>
+        public Action<Action>? UiInvoker { get; set; }
+
         private Task InvokeOnUiThreadAsync(Action action)
         {
-            // Use SynchronizationContext via Task to ensure UI thread update
             return Task.Run(() =>
             {
-                System.Windows.Application.Current?.Dispatcher.Invoke(action);
+                if (UiInvoker != null)
+                    UiInvoker(action);
+                else
+                    System.Windows.Application.Current?.Dispatcher.Invoke(action);
             });
         }