feat: add leaderboard filtering by language and time range

[ni2-vsv11]

Summary

Implements leaderboard filtering by language and time range for the DevTrack leaderboard.

Features Added

• Language dropdown filtering

• Time range filtering:

  • This Week
  • This Month
  • All Time

• URL query parameter synchronization

• localStorage persistence for filters

• Clear filters functionality

• Mobile responsive filter UI

• Filter-aware leaderboard API query handling

• Defensive filtering for missing language data

API Updates

Supports leaderboard queries like:

/api/leaderboard?lang=python&period=month

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Screenshots

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Verification

Tested:

• /leaderboard
• /leaderboard?lang=python
• /leaderboard?tab=commits&period=month
• /leaderboard?tab=streak&lang=typescript

Verified:

• rankings render correctly
• filters update without page reload
• URL reflects active filters
• leaderboard remains stable
• mobile responsiveness works
• npm run lint passes

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Notes

Local development database currently contains only one public leaderboard user, so local filtered leaderboard views may show limited results. The filtering system itself works correctly against the available dataset.

Closes #1045

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[github-actions]

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

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[ni2-vsv11]

@Priyanshu-byte-coder

Implemented the requested leaderboard filtering feature for #1045 and completed all requested functionality:

✅ Language dropdown filtering
✅ Time range filtering (Week / Month / All Time)
✅ URL query parameter synchronization
✅ localStorage persistence
✅ Clear filters support
✅ Mobile responsive filter UI
✅ Filter-aware leaderboard API handling
✅ Defensive handling for missing language data

Also resolved upstream merge/rebase conflicts and fixed the related Playwright strict-mode test issue to ensure CI passes cleanly.

Verification completed successfully:

• Build ✅
• Lint ✅
• Typecheck ✅
• Playwright E2E ✅

Would appreciate a review and merge when convenient.

[Priyanshu-byte-coder]

This PR has developed merge conflicts after recent merges to main. Please rebase onto the latest main branch, resolve all conflicts, and force-push. Once the PR is conflict-free it will be reviewed for merging.