fix(auth): return structured token_expired 401 error and add auto-red…

[Nareshkumawat-star]

Summary

Gracefully handles expired or revoked GitHub OAuth tokens across API route handlers (/api/metrics/*) by returning structured 401 JSON responses ({ error: "token_expired" }). Dashboard components detect token expiration, display a "Session expired — please sign in again" banner, and auto-redirect to /api/auth/signin after 3 seconds.

Closes #3244

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Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that changes existing behavior)
• 📝 Documentation update
• ♻️ Refactor / code cleanup (no functional change)
• ⚡ Performance improvement
• 🔒 Security fix
• 🧪 Tests only

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What Changed

• src/lib/github-fetch.ts: Returns GitHubAuthError on HTTP 401 responses.
• src/app/api/metrics/contributions/route.ts & src/app/api/metrics/prs/route.ts: Returns githubAuthErrorResponse() (HTTP 401 { error: "token_expired" }) when authentication is invalid or revoked.
• src/components/ContributionGraph.tsx: Catches 401 token_expired errors, shows a session expiration banner, and auto-redirects to /api/auth/signin after 3 seconds.

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How to Test

1. Sign in to DevTrack.
2. Revoke the DevTrack OAuth app token in GitHub Settings → Applications → Authorized OAuth Apps.
3. Trigger a metric fetch or time-range change on the dashboard.
4. Verify that the dashboard displays the "Your session expired — please sign in again" alert banner instead of freezing or throwing unhandled console errors.
5. Verify auto-redirection to /api/auth/signin after 3 seconds.

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Checklist

• Self-reviewed my own diff
• No unnecessary console.log or debug code
• Structured 401 error response code verified
• Auto-redirection user experience tested

[github-actions]

GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — please apply the appropriate labels before merging:

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• level:beginner — 20 pts
• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

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• quality:exceptional — ×1.5 multiplier

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[Nareshkumawat-star]

Hi @Priyanshu-byte-coder kindly review and merge it