Fix startup plugin paths for issue #331

[csharpfritz]

Summary

• anchor plugin-system paths to the ASP.NET Core content root through SharpSitePathProvider
• initialize PluginManager path resolution from the web app content root so default plugin installation works during OOTB setup
• add regression coverage ensuring Step2 does not directly inject IHandleFileStorage

Testing

• dotnet test .\tests\SharpSite.Tests.Plugins\SharpSite.Tests.Plugins.csproj --nologo --verbosity minimal
• dotnet test .\tests\SharpSite.Tests.Web\SharpSite.Tests.Web.csproj --nologo --verbosity minimal

[csharpfritz]

Closed as duplicate of #353. Both PRs implement identical solutions for issue #331. #353 is canonical with more comprehensive documentation.