feat: multi-account GitHub linking — link work/personal accounts & view combined stats (#2498)

[IshitaSingh0822]

Summary

Implements multi-account GitHub linking so developers can connect separate personal and work GitHub identities to a single DevTrack session, view combined contribution stats, and switch between accounts via the dashboard header switcher.

Closes #2498

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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/app/api/accounts/route.ts — GET endpoint to list all linked GitHub accounts
• src/app/api/accounts/link/route.ts — POST endpoint to initiate GitHub OAuth flow for a second account
• src/app/api/accounts/[githubLogin]/route.ts — DELETE endpoint to unlink an account by GitHub login
• src/app/settings/page.tsx — new /settings page with linked accounts management UI (add/remove, avatar display)
• src/components/AccountToggle.tsx — updated switcher with GitHub avatars, Combined view, and WCAG 2.1 AA keyboard accessibility
• supabase/migrations/20260617000000_add_linked_accounts_view.sql — linked_accounts view + RLS select/delete policies

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How to Test

1. Sign in to DevTrack with your primary GitHub account
2. Go to /settings and click "Link another GitHub account"
3. Complete the GitHub OAuth flow with a second account
4. Return to /settings — linked account appears with avatar and added date
5. Go to /dashboard — account switcher appears at the bottom of the header
6. Click the second account — all metric widgets re-fetch for that account
7. Click "Combined" — stats aggregate across both accounts
8. Click Remove on /settings — account disappears immediately
9. Tab through the switcher buttons and press Enter/Space to verify keyboard nav

Expected result: accounts link/unlink correctly, switcher updates all dashboard widgets, tokens never appear in plaintext in session or DB

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Checklist

• Linked the related issue above
• Self-reviewed my own diff
• No unnecessary console.log, debug code, or commented-out blocks
• npm run lint passes locally
• No TypeScript errors (npm run type-check)
• Added or updated tests where applicable
• Updated documentation / comments if behavior changed

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Accessibility (UI changes only)

• Keyboard navigation works correctly
• Color contrast meets WCAG AA standard
• ARIA labels / roles added where needed
• Tested on mobile / responsive layout

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Additional Context

Access tokens are stored AES-256-GCM encrypted using the existing encryptToken/decryptToken from src/lib/crypto.ts — same pattern as the existing user_github_accounts table. No breaking changes to existing single-account behaviour; fully additive.

[github-actions]

GSSoC Label Checklist 🏷️

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