fix: add missing let/const declarations and strict mode to sync-leaderboard.js (#60)

[rishab11250]

Description

Adds missing "use strict" directive and let declarations to variables in scripts/sync-leaderboard.js that were previously assigned without declaration, leaking to the global scope. Without strict mode, these implicit globals can cause silent data corruption (wrong scores, swapped rankings, duplicate entries) when the three parallel processing blocks (daily/weekly/monthly) share state — and no error is thrown.

Linked Issue

Fixes #60

Changes Made

• Added "use strict"; at the top of the file — makes accidental global assignment an immediate runtime error instead of silent corruption
• Added let to dailyData declaration (line 104)
• Added let to previousData declaration (line 108)
• Added let to weeklyData declaration (line 157)
• Added let to monthlyData declaration (line 216)

Subsequent reassignments of previousData (lines 161, 220) remain as-is — let allows reassignment within the same function scope.

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

Verification performed:

• node --check scripts/sync-leaderboard.js — Syntax OK
• npx prettier --check "frontend/**/*.{html,css,js}" "scripts/**/*.js" "server.js" — All files pass

Checklist

• My code follows the project's coding style
• I have formatted my code locally using Prettier
• 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 with no UI impact.

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