fix: Use user's local timezone for streak date calculations

[Pratikshya32]

Summary

Adds a timezone-aware date utility for streak calculations. Prevents users in non-UTC timezones from incorrectly losing their streak at midnight UTC.

Changes

• Added src/lib/dates/streakDateUtils.ts

cc @Priyanshu-byte-coder

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[github-actions]

GSSoC Label Checklist 🏷️

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

Difficulty (pick one):

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

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• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

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Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
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[Priyanshu-byte-coder]

This PR has merge conflicts with main. Please rebase:

git fetch origin
git rebase origin/main
# fix conflicts, then:
git push --force-with-lease

[Priyanshu-byte-coder]

This PR cannot be merged because it branches from a significantly outdated version of main and includes ~9400 deletions of code that was already updated separately. The actual feature/fix in this PR is valuable though!

To get your contribution merged:

1. Sync your fork: git fetch upstream && git rebase upstream/main
2. Create a new branch from the updated main: git checkout -b your-feature-name origin/main
3. Cherry-pick or re-apply only your actual changes
4. Open a new focused PR with just your changes

Labeling your contribution for GSSoC scoring.

[Pratikshya32]

Hi @Priyanshu-byte-coder! Thank you for the detailed review! I have addressed all the concerns:

What was fixed

The PR branch ix/streak-timezone-fix has been rebased and cleaned up:

• Synced fork with the latest upstream/main
• Created a fresh branch from updated main
• Kept only the actual feature change: src/lib/dates/streakDateUtils.ts
• Removed all the ~9400 spurious deletions that came from the outdated fork base

Changes in this PR (now clean)

Only 1 file added — src/lib/dates/streakDateUtils.ts:

• getLocalDateString(timezone) — Get current date in user's local timezone
• utcToLocalDate(timestamp, timezone) — Convert UTC to local date string
• �reConsecutiveDays(date1, date2) — Timezone-aware consecutive day check
• calculateStreak(dates, userTimezone) — Timezone-aware streak calculation

This should now be merge-ready. Please take a look and let me know if any further changes are needed!

[github-actions]

🎉 Merged! Thanks for contributing to DevTrack.

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