ControlCatalog: default to the Fluent2 theme

The catalog now starts under Fluent2; v1 Fluent and Simple remain
available from the settings drawer as before.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

samples/ControlCatalog/App.xaml.cs

@@ -37,7 +37,7 @@ namespace ControlCatalog
             _colorPickerFluent = (IStyle)Resources["ColorPickerFluent"]!;
             _colorPickerSimple = (IStyle)Resources["ColorPickerSimple"]!;
 
-            SetCatalogThemes(CatalogTheme.Fluent);
+            SetCatalogThemes(CatalogTheme.Fluent2);
         }
 
         public override void OnFrameworkInitializationCompleted()