feat: add PackageIcon and PackageReadmeFile for NuGet packaging

- Copy GeneralUpdate128.png to imgs/
- Add <PackageIcon> and <PackageReadmeFile> to csproj
- Add <None> Include to pack icon into NuGet package

Co-Authored-By: Claude <noreply@anthropic.com>

Author: JusterZhu

imgs/GeneralUpdate128.png

@@ -8,8 +8,17 @@
     <Authors>GeneralLibrary</Authors>
     <Description>GeneralUpdate.Maui.Android is a UI-less Android auto-update core component for .NET MAUI.</Description>
     <GeneratePackageOnBuild>False</GeneratePackageOnBuild>
+    <PackageIcon>GeneralUpdate128.png</PackageIcon>
+    <PackageReadmeFile>README.md</PackageReadmeFile>
   </PropertyGroup>
 
+  <ItemGroup>
+    <None Include="../../imgs/GeneralUpdate128.png">
+      <Pack>True</Pack>
+      <PackagePath>\</PackagePath>
+    </None>
+  </ItemGroup>
+
   <ItemGroup>
     <PackageReference Include="Microsoft.Extensions.DependencyInjection.Abstractions" Version="10.0.0" />
     <PackageReference Include="Microsoft.Extensions.Http" Version="10.0.0" />