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/Properties/launchSettings.json

@@ -1,12 +1,15 @@
 {
   "profiles": {
     "ImageCatalog (WinForms)": {
-      "commandName": "Project",
-      "commandLineArgs": ""
+      "commandName": "Project"
     },
     "ImageCatalog (WPF)": {
       "commandName": "Project",
       "commandLineArgs": "--wpf"
+    },
+    "ImageCatalog (Avalonia)": {
+      "commandName": "Project",
+      "commandLineArgs": "--avalonia"
     }
   }
-}
+}