Devolutions transition and release automation updates

[Marc-André Moreau (mamoreau-devolutions)]

Summary

• update the repository, documentation, and project metadata for the Devolutions transition
• add the new build and release pipeline, including signing, packaging, draft release support, and WinGet bundle fetching
• switch the app updater to the Devolutions productinfo source with fallback to the legacy GitHub-based logic when productinfo is unavailable
• add manual updater test fixtures and documentation for the new productinfo and fallback flows
• remove legacy bundled release artifacts and in-app announcements that are no longer part of the release path

Key details

• the release workflow now supports manual draft GitHub Releases so productinfo can be updated before publishing
• the updater is more resilient for first-release scenarios because it no longer hard-fails when productinfo does not yet contain UniGetUI metadata
• release/build scripts are now aligned with the Devolutions signing and packaging flow

Validation

• dotnet build src/UniGetUI/UniGetUI.csproj -nologo /p:Platform=x64 -v q
• manual review of .github/workflows/build-release.yml
• manual review of updater fallback documentation in testing/UPDATE-TESTING.md

Notes

• this branch includes the Devolutions transition and release-pipeline stack, not only the latest updater/workflow tweaks