Add Nightscout WebSocket support for real-time data updates

[bjorkert]

Summary

• Adds optional Socket.IO WebSocket connection to Nightscout for real-time push notifications of data changes
• WebSocket delta events selectively trigger HTTP fetches only for changed data types (BG, treatments, device status, profile), reducing unnecessary network requests
• Polling intervals are extended (3x for BG, 5x for treatments/device status, 3x for profile) while WebSocket is authenticated, with immediate fallback to normal polling if the connection drops
• WebSocket is foreground-only: it disconnects when the app moves to the background and reconnects on return, so the persistent connection only runs while you're using the app
• Toggle in Nightscout settings defaults to on; info sheet describes the foreground/background behavior

[bjorkert]

[bjorkert]

Addressed both review findings:

Stale WebSocket session on config change (high): updateStatusLabel() now calls disconnect() when URL/token validation fails, so the old socket can't keep streaming from the previous server. Additionally, removeAllHandlers() is now called before disconnect() in the teardown path — this prevents intentional disconnects from firing the .disconnect event handler, which would post a refresh and potentially reconnect to the invalid URL.

Staleness timer (medium): Removed entirely. The extended polling intervals (3x for BG/device status, 5x for treatments, 3x for profile) already continue running as a safety net. If the socket authenticates but no data flows, polling still fetches via HTTP at the extended cadence. The staleness timer was redundant and had a bug where it never armed if no dataUpdate was received.