feat: implement incremental streak tracking and backfill initialization

[Yashaswini-K-P]

Description

Implemented incremental streak tracking to enhance user engagement. This PR introduces a backend utility to calculate streak history and updates the sync process to use constant-time (O(1)) logic, avoiding expensive full-history scans during every 15-minute sync.

Linked Issue

Related to #51

Changes Made

• Added scripts/backfill-streaks.js to calculate and initialize currentStreak, longestStreak, and streakLastUpdated for existing users.
• Updated sync-leaderboard.js to include incremental streak logic (Step A: Check for broken streak, Step B & C: Detect activity and update streak).
• Added logic to persist streak metadata in user-specific JSON files.

Type of Change

• Bug fix
• New feature
• UI/Visual update
• Documentation update
• Refactor

Testing

• Tested locally
• Tested on mobile viewport (if applicable)
• No console errors introduced

Checklist

• My code follows the project's coding style
• I have formatted my code locally by running npx prettier --write . before submitting
• I am submitting my PR from a dedicated feature/* branch, not the main branch
• I have performed a self-review of my code
• My changes generate no new warnings or errors
• I have updated documentation if required
• I have linked the relevant issue

Screenshots / Screen Recording

N/A

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