feat: update demo.

demo/Ursa.Demo/Dialogs/DialogWithAction.axaml

@@ -5,6 +5,7 @@
              xmlns:local="clr-namespace:Ursa.Demo.Dialogs"
              x:DataType="local:DialogWithActionViewModel"
              x:CompileBindings="True"
+             Background="{DynamicResource SemiYellow1}"
              mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
              x:Class="Ursa.Demo.Dialogs.DialogWithAction">
     <StackPanel Margin="24">

demo/Ursa.Demo/Dialogs/PlainDialog.axaml

@@ -3,6 +3,9 @@
              xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
              xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
              mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
+             xmlns:local="clr-namespace:Ursa.Demo.Dialogs"
+             x:DataType="local:PlainDialogViewModel"
+             x:CompileBindings="True"
              x:Class="Ursa.Demo.Dialogs.PlainDialog">
     <StackPanel>
         <Calendar SelectedDate="{Binding Date}" ></Calendar>

demo/Ursa.Demo/Dialogs/PlainDialogViewModel.cs

@@ -1,8 +1,15 @@
-using CommunityToolkit.Mvvm.ComponentModel;
+using System;
+using CommunityToolkit.Mvvm.ComponentModel;
 
 namespace Ursa.Demo.Dialogs;
 
 public class PlainDialogViewModel: ObservableObject
 {
-    
+    private DateTime? _date;
+
+    public DateTime? Date
+    {
+        get => _date;
+        set => SetProperty(ref _date, value);
+    }
 }