Add NuGet packaging metadata

[miiitch]

Goal

Prepare the Razor Class Library for publication to NuGet.org.

Tasks

• Add NuGet metadata to src/MudBlazorExtended/MudBlazorExtended.csproj:
  • PackageId, Version (start at 0.1.0), Authors, Description, PackageTags
  • RepositoryUrl, PackageLicenseExpression (MIT)
  • PackageReadmeFile pointing to README.md
  • PackageIcon (add icon.png asset)
  • GeneratePackageOnBuild = false
• Add a LICENSE file (MIT)
• Verify dotnet pack produces a valid .nupkg

Depends on

#1

Acceptance criteria

• dotnet pack src/MudBlazorExtended succeeds without warnings
• Inspecting the .nupkg shows correct metadata