feat: improve sync code

[caiodomingues]

Summary

• Re-enable auto-sync that was disabled due to ~400k GitHub API requests/hour
• Add 3-layer safeguards to prevent API abuse: client-side cooldown (30min), server-side cooldown (5min), and GitHub
  rate limit pre-check (<50 remaining → skip)
• Replace window.location.reload() with in-place leaderboard refresh
• Add BroadcastChannel to prevent duplicate syncs across browser tabs

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[Copilot]

Pull request overview

This PR re-enables the auto-sync feature for XP data that was previously disabled due to excessive GitHub API requests (~400k/hour). It introduces a 3-layer safeguard system: client-side localStorage-based cooldown (30 min), server-side database cooldown (5 min), and GitHub rate limit pre-check (skip if <50 remaining). It also replaces the page reload approach with in-place leaderboard refresh and adds cross-tab sync deduplication via BroadcastChannel.

Changes:

• Added rate limit pre-check in syncUserData to skip syncs when GitHub API quota is low
• Added server-side cooldown in the force-sync API route to return cached data when the last update was less than 5 minutes ago
• Replaced the commented-out auto-sync with a properly guarded implementation using client-side cooldown, cross-tab deduplication, and in-place leaderboard refresh

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
lib/xp-calculator.ts	Adds GitHub rate limit pre-check before performing sync API calls
app/api/force-sync/route.ts	Adds server-side 5-minute cooldown using lastXpUpdate field
app/page.tsx	Replaces disabled auto-sync with guarded implementation (30-min client cooldown, BroadcastChannel cross-tab dedup, in-place leaderboard refresh)

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