feat: add demo.

demo/Ursa.Demo/Pages/PopConfirmDemo.axaml

@@ -0,0 +1,19 @@
+<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:u="https://irihi.tech/ursa"
+             xmlns:viewModels="clr-namespace:Ursa.Demo.ViewModels"
+             mc:Ignorable="d" d:DesignWidth="800"
+             d:DesignHeight="450"
+             x:DataType="viewModels:PopConfirmDemoViewModel"
+             x:Class="Ursa.Demo.Pages.PopConfirmDemo">
+    <StackPanel HorizontalAlignment="Left">
+        <u:PopConfirm PopupHeader="Header" PopupContent="Content" 
+                     ConfirmCommand="{Binding Path=ConfirmCommand}"
+                     CancelCommand="{Binding Path=CancelCommand}"
+                     >
+            <Button Content="Hello World"></Button>
+        </u:PopConfirm>
+    </StackPanel>
+</UserControl>

demo/Ursa.Demo/Pages/PopConfirmDemo.axaml.cs

@@ -0,0 +1,25 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Markup.Xaml;
+using Ursa.Controls;
+using Ursa.Demo.ViewModels;
+
+namespace Ursa.Demo.Pages;
+
+public partial class PopConfirmDemo : UserControl
+{
+    public PopConfirmDemo()
+    {
+        InitializeComponent();
+    }
+
+    protected override void OnAttachedToVisualTree(VisualTreeAttachmentEventArgs e)
+    {
+        base.OnAttachedToVisualTree(e);
+        if (this.DataContext is not PopConfirmDemoViewModel vm) return;
+        var manager = WindowToastManager.TryGetToastManager(TopLevel.GetTopLevel(this), out var m)
+            ? m
+            : new WindowToastManager(TopLevel.GetTopLevel(this));
+        vm.ToastManager = manager;
+    }
+}