Add project files.

App/WeddingBuilder/LifeCycles/StartedHandler.cs

@@ -0,0 +1,27 @@
+using Avalonia;
+using Avalonia.Controls.ApplicationLifetimes;
+using TheXamlGuy.Framework.Core;
+
+namespace Builder;
+
+public class StartedHandler : IMediatorHandler<Started>
+{
+    private readonly MainWindow window;
+    private readonly MainWindowViewModel viewModel;
+
+    public StartedHandler(MainWindow window, 
+        MainWindowViewModel viewModel)
+    {
+        this.window = window;
+        this.viewModel = viewModel;
+    }
+
+    public void Handle(Started request)
+    {
+        window.DataContext = viewModel;
+        if (Application.Current?.ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
+        {
+            desktop.MainWindow = window;
+        }
+    }
+}