Refactor application to remove Windows Forms dependencies and transition to Avalonia UI

- Deleted MainWindow.xaml.cs, which contained the WPF implementation of the main window.
- Updated Program.cs to remove Windows Forms initialization and support only Avalonia UI.
- Removed Windows Forms specific code from ViewModelBase, including control marshalling logic.

imagecatalog/DataModel.cs

@@ -15,9 +15,6 @@ using System.Runtime.InteropServices;
 using System.Text;
 using System.Threading;
 using System.Threading.Tasks;
-#if WINDOWS
-using System.Windows.Forms;
-#endif
 using System.Windows.Input;
 using AutoMapper;
 using MaddoShared;
@@ -167,8 +164,7 @@ namespace ImageCatalog_2
         }
 
         /// <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.
+        /// Optional UI-thread invoker set by the active UI layer.
         /// </summary>
         public Action<Action>? UiInvoker { get; set; }
 
@@ -179,7 +175,7 @@ namespace ImageCatalog_2
                 if (UiInvoker != null)
                     UiInvoker(action);
                 else
-                    System.Windows.Application.Current?.Dispatcher.Invoke(action);
+                    action();
             });
         }
 
@@ -1886,11 +1882,7 @@ namespace ImageCatalog_2
         public event EventHandler<string?>? LoadSettingsRequested;
         public event EventHandler? SelectColorRequested;
         // Request that the View shows a message to the user (message, caption, icon)
-#if WINDOWS
-        public event EventHandler<Tuple<string, string, MessageBoxIcon>>? ShowMessageRequested;
-#else
         public event EventHandler<Tuple<string, string, int>>? ShowMessageRequested;
-#endif
         public event EventHandler? SelectTransparentColorRequested;
 
         private void SelectSourceFolder(object parameter)