fix: support WASM.

demo/Ursa.Demo/App.axaml.cs

@@ -0,0 +1,35 @@
+using Avalonia;
+using Avalonia.Controls.ApplicationLifetimes;
+using Avalonia.Markup.Xaml;
+using Ursa.Demo.ViewModels;
+using Ursa.Demo.Views;
+
+namespace Ursa.Demo;
+
+public partial class App : Application
+{
+    public override void Initialize()
+    {
+        AvaloniaXamlLoader.Load(this);
+    }
+
+    public override void OnFrameworkInitializationCompleted()
+    {
+        if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
+        {
+            desktop.MainWindow = new MainWindow()
+            {
+                DataContext = new MainViewViewModel(),
+            };
+        }
+        else if (ApplicationLifetime is ISingleViewApplicationLifetime singleView)
+        {
+            singleView.MainView = new MainView()
+            {
+                DataContext = new MainViewViewModel(),
+            };
+        }
+
+        base.OnFrameworkInitializationCompleted();
+    }
+}