fix: stabilize GitHub OAuth sign-in flow (Closes #821)

[orgy272]

Summary

Fixes GitHub OAuth sign-in failures where users hit a 404 path and adds graceful callback error handling.

What changed

• Added a single OAuth authorize endpoint helper that respects VITE_API_URL when present.
• Updated all GitHub sign-in entry points to use that helper for fallback redirect:
  • Navbar
  • AuthGuard
  • HeroSection
  • BountyDetail sign-in link
• Hardened callback handling on /auth/github/callback:
  • Explicit error UI (instead of silent redirect) for canceled/invalid callback states
  • Clear messages for expired/invalid code, invalid state/session, rate limits, and server errors
  • Retry action wired to OAuth authorize endpoint
• Added unit tests for auth API helper fallback behavior.

Validation

• npm run test -- auth-api.test.ts apiClient.test.ts (25 passing)

Closes #821

Wallet: 61FYMEPXMe73ypR53wMAR7PYAWHhZWKFJMNKnG9NwoW

[orgy272]

I pushed a follow-up fix to address frontend baseline CI blockers on this branch:

• restored tracked frontend libs (frontend/src/lib/animations.ts, frontend/src/lib/utils.ts)
• fixed .gitignore rule from lib/ to /lib/ so frontend/src/lib is not ignored

This should clear the TypeScript/Build module-not-found failures.
If backend jobs still fail, they appear to be repository-level CI expectations for a backend/ directory that is currently absent in this codebase.

[orgy272]

CI is green for project checks after latest fixes; remaining red is AI Code Review / trigger-review failing with GitHub API 403 rate-limit inside repository automation (API rate limit exceeded for user ID ...).

This failure is external to PR code changes and requires rate-limit reset or maintainer/admin rerun rights.