[Fix] #1312 Streak logic duplication resolved.

[palxsh01]

Summary

Extracts the duplicated streak calculation algorithm into a single shared module
at src/lib/streak.ts, removing ~230 lines of copy-pasted logic spread across
6 source files. Also fixes two sil consolidation.

Closes #1312

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Type of Change

• Bug fix
• New feature
• Documentation update
• Refactor / code cleanup

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Changes Made

• Created src/lib/streak.ts exposing two shared exports:
  • calculateStreakFromDates(activeDates, freezeDates?) — full calculation with freeze date support
  • calculateCurrentStreak(dates) — lightweight wrapper returning only the current streak number
• Removed inline streak logic from 6 source files: streak/route.ts, streak-shield/route.ts, public-profile-data.ts, compare/route.ts, leaderboard/route.ts, weekly-summary/route.ts
• Updated calculateStreakFromDates.test.ts and leaderboard-streak.test.ts to import from the shared module instead of maintaining local copies
• Fixed silent bug in compare/route.ts where longestStreak was never tracked — endpoint returned only current streak
• Fixed behavioral divergence in streak-shield/route.ts where dateDiffDays used Date.UTC() instead of ISO string truncation, causing off-by-one results for users near UTC midnight

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How to Test

1. Run npm run type-check — should pass with no errors
2. Run npm test — all 285 tests should pass
3. Run npm run lint — should pass
4. Manually verify that /api/metrics/streak, /api/badge/streak-shield, and
   /api/public/[username] return consistent streak values for the same GitHub user

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Screenshots (if UI change)

N/A — backend refactor only, no UI changes.

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Checklist

• Linked issue in summary
• npm run lint passes locally
• No TypeScript errors (npm run type-check)
• Self-reviewed the diff
• Added/updated tests if applicable

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Accessibility Checklist

• Proper keyboard navigation tested
• Responsive UI verified
• Accessibility labels added where needed

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Additional Notes

All existing tests continue to pase follows the same commentingconventions as the rest of src/lib/ — inline // comments for non-obvious constraints, a short JSDoc block on the exported function.

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[palxsh01]

Branch was updated and multiple merge conflicts have arised. Resolving merge conflicts and will open a new PR for a clean merge into main.