feat: make UrsaView base.

demo/Ursa.Demo/App.axaml.cs

@@ -1,4 +1,5 @@
 using Avalonia;
+using Avalonia.Controls;
 using Avalonia.Controls.ApplicationLifetimes;
 using Avalonia.Markup.Xaml;
 using Ursa.Demo.ViewModels;
@@ -17,8 +18,9 @@ public partial class App : Application
     {
         if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
         {
-            desktop.MainWindow = new MainWindow()
+            desktop.MainWindow = new Window()
             {
+                Content = new SingleView(),
                 DataContext = new MainViewViewModel(),
             };
         }