fix: calculate daily streak using user local timezone

[jahnavi-karanwal]

Summary

Fixed timezone-related streak calculation issues in src/app/api/metrics/streak/route.ts by ensuring commit dates and streak boundaries are calculated using the user's local timezone instead of raw UTC dates.

Closes #897

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Type of Change

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

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Changes Made

• Updated fetchActiveDates() to convert GitHub commit timestamps into the user's local timezone before extracting and storing the date.
• Fixed date normalization logic to prevent commits near midnight from being assigned to the wrong calendar day.
• Updated calculateStreakFromDates() to derive today and yesterday using the user's local timezone rather than the server's UTC date.
• Ensured streak calculations remain accurate for users across all time zones, including positive and negative UTC offsets.
• Eliminated streak breaks caused by timezone differences between GitHub timestamps and server-side date calculations.

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Checklist

• Linked issue in summary
• Self-reviewed the diff
• No breaking changes introduced
• Added/updated tests if applicable

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[github-actions]

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