Backport #5311 (forum login loop fix) to release/v1.2

[Ma77Ball]

Task Summary

Backport #5311 ("fix(frontend): stop forum login from looping when Flarum auth fails") to the release/v1.2 branch for the v1.2 release.

Requested by @xuang7. The PR was merged before the release/v1.2 label existed, so auto-backport did not trigger. A manual backport PR (#5615) cherry-picks the merged squash commit (af664995) onto release/v1.2.

Task Type

• Refactor / Cleanup
• DevOps / Deployment / CI
• Testing / QA
• Documentation
• Performance
• Other

[Ma77Ball]

/take

[xuang7]

Closing this issue and linking the backport PR to the original issue.