feat: initialize projects.

demo/Ursa.Demo/Views/MainWindow.axaml

@@ -0,0 +1,26 @@
+<Window
+    x:Class="Ursa.Demo.Views.MainWindow"
+    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:vm="using:Ursa.Demo.ViewModels"
+    Title="Ursa.Demo"
+    d:DesignHeight="450"
+    d:DesignWidth="800"
+    Icon="/Assets/avalonia-logo.ico"
+    mc:Ignorable="d">
+
+    <Design.DataContext>
+        <vm:MainWindowViewModel />
+    </Design.DataContext>
+
+    <StackPanel Spacing="20">
+        <TextBlock
+            HorizontalAlignment="Center"
+            VerticalAlignment="Center"
+            Text="{Binding Greeting}" />
+        <Button Content="Hello World" />
+    </StackPanel>
+
+</Window>

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

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+using Avalonia.Controls;
+
+namespace Ursa.Demo.Views;
+
+public partial class MainWindow : Window
+{
+    public MainWindow()
+    {
+        InitializeComponent();
+    }
+}