Fix inconsistent timezone handling in contributions API and StreakCalendar

[saurabhhhcodes]

Bug 1: Mixed timezone handling in contributions API route

fetchContributionsForAccount used toLocalDateStr() (local timezone) to format the since date for the GitHub API query, while every other route uses toISOString().slice(0, 10) (UTC). For users in negative UTC offsets this caused a one-day shift in the query window, making streak and contribution data disagree on which dates to include. Fixed by removing toLocalDateStr and using since.toISOString().slice(0, 10) consistently with the rest of the codebase.

Bug 2: StreakCalendar used local dates to look up UTC-keyed contribution data

The StreakCalendar component used toLocalDateStr() (local timezone) to construct the lookup key into the contribution data, but the data keys come from GitHub's API which always returns UTC dates (item.commit.author.date.slice(0, 10)). This caused commits to appear on wrong calendar cells for users whose local date differs from UTC. Fixed by renaming to toUtcDateStr and using toISOString().slice(0, 10) for both lookup and isToday comparison.

[github-actions]

GSSoC Label Checklist 🏷️

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

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• level:critical — 80 pts

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