[Marc-André Moreau (mamoreau-devolutions)]

• I have read the contributing guidelines, and I agree with the Code of Conduct.
• Have you checked that there aren't other open pull requests for the same changes?
• Have you tested that the committed code can be executed without errors?
• This PR is not composed of garbage changes used to farm GitHub activity to enter potential Crypto AirDrops.
  Any user suspected of farming GitHub activity with crypto purposes will get banned. Submitting broken code wastes the contributors' time, who have to spend their free time reviewing, fixing, and testing code that does not even compile breaks other features, or does not introduce any useful changes. I appreciate your understanding.

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This fixes the GitHub cloud backup login break introduced during the repository migration.

Changes included:

• wire the OAuth client ID and secret into the release workflow using the normalized UNIGETUI_GITHUB_* secret names
• fail early in CI when those secrets are missing
• harden the GitHub login flow so it fails clearly when OAuth credentials are unset or the loopback callback never arrives
• update the affected login/backup help and privacy links away from the old marticliment.com targets

Validation:

• dotnet build d:/dev/unigetui-github-app/src/UniGetUI/UniGetUI.csproj -p:Platform=x64 -p:Configuration=Debug -nologo -v minimal

Manual OAuth end-to-end validation still requires a valid GitHub OAuth app client ID and secret.

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Closes #4461

[TPS]

#4788 reports that this still isn't working.…