feat: add default template.

src/Ursa/Controls/MessageBox/MessageBox.cs

@@ -1,12 +1,28 @@
 using Avalonia;
 using Avalonia.Controls.ApplicationLifetimes;
 
-namespace Ursa.Controls.MessageBox;
+namespace Ursa.Controls;
 
 public static class MessageBox
 {
     public static async Task ShowAsync(string message)
     {
-        var messageWindow = new MessageBoxWindow();
+        var messageWindow = new MessageBoxWindow()
+        {
+            Content = message
+        };
+        var lifetime = Application.Current?.ApplicationLifetime;
+        if (lifetime is IClassicDesktopStyleApplicationLifetime classLifetime)
+        {
+            var main = classLifetime.MainWindow;
+            if (main is null)
+            {
+                messageWindow.Show();
+            }
+            else
+            {
+                await messageWindow.ShowDialog(main);
+            }
+        }
     }
 }