- Add checkForUpdates() with 10-minute cached GitHub API check
- Add fetchLatestCommitInfo() to get latest commit SHA + message from GitHub API
- Save deployed commit SHA to writable/deployed_commit.txt after each deploy
- Invalidate update cache on successful deploy so next check is always fresh
- Button turns yellow + red NEW badge when update available
- Show commit message and short SHA diff (deployed → latest) in alert
- Show green 'Up to date' with deployed SHA when already current
- GitHub API rate limit safe: checks once per 10 mins via file cache