Fix GitHub cloud backup OAuth configuration

[Marc-André Moreau (mamoreau-devolutions)]

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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