Enable Blazor Explorer on Linux

[skidzo]

Summary

• enable the Blazor Explorer build on Linux by retargeting the Blazor dependency graph to net8.0 and removing Windows-only project references from that path
• add narrowly scoped compatibility stubs for desktop-only OpenID, signature, certificate-selection, and UI workflows; those workflows remain explicitly unavailable on Linux
• replace the Windows-specific BuildDate pre-build command with a cross-platform MSBuild task
• preserve the Windows desktop projects and their behavior outside the Linux Blazor dependency graph

Why

The Blazor Explorer can provide a browser-based Package Explorer surface on Linux and CI hosts. The previous project graph required Windows-targeting assemblies even when no desktop UI was used.

Validation

• /home/johannes/Dokumente/hub/IDTA_submodel_metal_additive_manufacturing/tools/package-explorer/.dotnet/dotnet build src/BlazorExplorer/BlazorExplorer.csproj -c Debug --no-incremental -m:1 -nodeReuse:false --nologo -v:minimal -clp:ErrorsOnly
• result: build succeeded (0 errors; existing upstream warnings remain)
• started the generated net8.0 Blazor application on Linux without a GUI
• GET / returned HTTP 200
• GET /_content/BlazorInputFile/inputfile.js returned HTTP 200

Scope

This is intentionally limited to the Blazor/Linux path. It does not change the Windows desktop Package Explorer, introduce a GUI requirement, or claim that desktop-only authentication, signing, or certificate-selection workflows are available on Linux.