feat: add overlay dialog host back to single view mode.

demo/Ursa.Demo/App.axaml.cs

@@ -24,7 +24,7 @@ public partial class App : Application
         }
         else if (ApplicationLifetime is ISingleViewApplicationLifetime singleView)
         {
-            singleView.MainView = new MainView()
+            singleView.MainView = new SingleView()
             {
                 DataContext = new MainViewViewModel(),
             };

demo/Ursa.Demo/Views/SingleView.axaml

@@ -0,0 +1,14 @@
+<UserControl xmlns="https://github.com/avaloniaui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+             xmlns:views="clr-namespace:Ursa.Demo.Views"
+             xmlns:u="https://irihi.tech/ursa"
+             mc:Ignorable="d" d:DesignWidth="800"
+             d:DesignHeight="450"
+             x:Class="Ursa.Demo.Views.SingleView">
+    <Panel>
+        <views:MainView/>
+        <u:OverlayDialogHost/>
+    </Panel>
+</UserControl>

demo/Ursa.Demo/Views/SingleView.axaml.cs

@@ -0,0 +1,13 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Markup.Xaml;
+
+namespace Ursa.Demo.Views;
+
+public partial class SingleView : UserControl
+{
+    public SingleView()
+    {
+        InitializeComponent();
+    }
+}