feat: cross platform projects.

demo/Ursa.Demo.Desktop/App.axaml

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+<Application
+    x:Class="Ursa.Demo.Desktop.App"
+    xmlns="https://github.com/avaloniaui"
+    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
+    <Application.Styles>
+        <StyleInclude Source="avares://Semi.Avalonia/Themes/Index.axaml" />
+        <StyleInclude Source="avares://Ursa.Themes.Semi/Index.axaml" />
+    </Application.Styles>
+</Application>

demo/Ursa.Demo.Desktop/App.axaml.cs

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+using Avalonia;
+using Avalonia.Controls.ApplicationLifetimes;
+using Avalonia.Markup.Xaml;
+using Ursa.Demo.ViewModels;
+using Ursa.Demo.Views;
+
+namespace Ursa.Demo.Desktop;
+
+public partial class App : Application
+{
+    public override void Initialize()
+    {
+        AvaloniaXamlLoader.Load(this);
+    }
+
+    public override void OnFrameworkInitializationCompleted()
+    {
+        if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
+        {
+            // Line below is needed to remove Avalonia data validation.
+            // Without this line you will get duplicate validations from both Avalonia and CT
+            desktop.MainWindow = new MainWindow
+            {
+                DataContext = new MainWindowViewModel(),
+            };
+        }
+
+        base.OnFrameworkInitializationCompleted();
+    }
+}

demo/Ursa.Demo.Desktop/Program.cs

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+using System;
+using Avalonia;
+using Avalonia.Dialogs;
+using Avalonia.Media;
+
+namespace Ursa.Demo.Desktop;
+
+class Program
+{
+    // Initialization code. Don't use any Avalonia, third-party APIs or any
+    // SynchronizationContext-reliant code before AppMain is called: things aren't initialized
+    // yet and stuff might break.
+    [STAThread]
+    public static void Main(string[] args) => BuildAvaloniaApp()
+        .With(new FontManagerOptions
+        {
+            FontFallbacks = new[]
+            {
+                new FontFallback
+                {
+                    FontFamily = new FontFamily("Microsoft YaHei")
+                }
+            }
+        })
+        .StartWithClassicDesktopLifetime(args);
+
+    // Avalonia configuration, don't remove; also used by visual designer.
+    public static AppBuilder BuildAvaloniaApp()
+        => AppBuilder.Configure<App>()
+            .UseManagedSystemDialogs()
+            .UsePlatformDetect()
+            .With(new Win32PlatformOptions())
+            .LogToTrace();
+}

demo/Ursa.Demo.Desktop/Ursa.Demo.Desktop.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+    <PropertyGroup>
+        <OutputType>WinExe</OutputType>
+        <!--If you are willing to use Windows/MacOS native APIs you will need to create 3 projects.
+        One for Windows with net7.0-windows TFM, one for MacOS with net7.0-macos and one with net7.0 TFM for Linux.-->
+        <TargetFramework>net6.0</TargetFramework>
+        <Nullable>enable</Nullable>
+        <BuiltInComInteropSupport>true</BuiltInComInteropSupport>
+    </PropertyGroup>
+
+    <PropertyGroup>
+        <ApplicationManifest>app.manifest</ApplicationManifest>
+    </PropertyGroup>
+
+    <ItemGroup>
+        <PackageReference Include="Avalonia.Desktop" Version="$(AvaloniaVersion)"/>
+        <!--Condition below is needed to remove Avalonia.Diagnostics package from build output in Release configuration.-->
+        <PackageReference Condition="'$(Configuration)' == 'Debug'" Include="Avalonia.Diagnostics" Version="$(AvaloniaVersion)"/>
+    </ItemGroup>
+
+    <ItemGroup>
+        <ProjectReference Include="..\Ursa.Demo\Ursa.Demo.csproj"/>
+    </ItemGroup>
+
+    <ItemGroup>
+      <Compile Update="App.axaml.cs">
+        <DependentUpon>App.axaml</DependentUpon>
+        <SubType>Code</SubType>
+      </Compile>
+    </ItemGroup>
+</Project>

demo/Ursa.Demo.Desktop/app.manifest

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+<?xml version="1.0" encoding="utf-8"?>
+<assembly manifestVersion="1.0" xmlns="urn:schemas-microsoft-com:asm.v1">
+  <!-- This manifest is used on Windows only.
+       Don't remove it as it might cause problems with window transparency and embeded controls.
+       For more details visit https://learn.microsoft.com/en-us/windows/win32/sbscs/application-manifests -->
+  <assemblyIdentity version="1.0.0.0" name="Ursa.Demo.Desktop"/>
+
+  <compatibility xmlns="urn:schemas-microsoft-com:compatibility.v1">
+    <application>
+      <!-- A list of the Windows versions that this application has been tested on
+           and is designed to work with. Uncomment the appropriate elements
+           and Windows will automatically select the most compatible environment. -->
+
+      <!-- Windows 10 -->
+      <supportedOS Id="{8e0f7a12-bfb3-4fe8-b9a5-48fd50a15a9a}" />
+    </application>
+  </compatibility>
+</assembly>