Fix delayed WebSocket reconnection when screen is turned off

[Copilot]

Mobile Chrome aggressively throttles setTimeout when the page is hidden (screen off), causing reconnection delays of minutes instead of seconds.

Changes

• Visibility API listener: On visibilitychange → visible, immediately calls reconnect() if the socket is not OPEN, bypassing any throttled timers
• Cleanup: Removes the listener on close() to prevent reconnection after intentional closure
• Guard: Only runs in browser environments (typeof document !== 'undefined'); no-op in Node.js

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}