fix: prevent data corruption in sync-leaderboard.js with atomic writes

[rishab11250]

Description

Prevent permanent data loss by replacing all in-place fs.writeFileSync() calls with an atomicWrite() helper that writes to a temporary file first, then atomically renames. If the process crashes mid-write (GitHub Actions runner preemption, OOM, timeout), the original file is never touched.

Linked Issue

Fixes #182

Changes Made

• atomicWrite() helper — writes JSON.stringify(data, null, 2) to a .tmp.{pid}.{timestamp} file on the same directory, then fs.renameSync() to the target path. renameSync is atomic on both POSIX and NTFS (same filesystem).
• Startup cleanup — scans data/ and data/daily/ for orphaned *.tmp.* files from previous crashes and deletes them.
• 8 writes converted — daily snapshot, overall.json, daily.json, weekly.json, monthly.json, changes.json, last-sync.json, and user history files (updateUserHistory).
• No formatting changes — JSON.stringify(data, null, 2) is preserved inside the helper. Output files are byte-for-byte identical.

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

• node --check scripts/sync-leaderboard.js — syntax check ✅

• npx prettier --check scripts/sync-leaderboard.js — formatting check ✅

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 — Backend-only change.

[github-actions]

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