fix: validate sidebar width, add role=alert, keyboard nav, aria-labels, periodic health check

[saurabhhhcodes]

Fixes #140, #141, #142, #143, #144

[Mananwebdev160408]

Closing as duplicate — the sidebar width validation fix was merged via #152, the role=alert fix via #151, aria-labels via #149, keyboard nav via #150, and health polling via #148. All your fixes went in through separate focused PRs. Thank you! 🙏