Use a WebSocket to display the firewall connection status to the UI in real-time

[Fraccs]

Use a WebSocket to display the firewall connection status to the UI in real-time

Issue

The firewall "connection status" (whether the firewall is connected or not to the socket) is currently fetched (GET /api/firewall) by the frontend each 5 seconds (which is not ideal for a variety of reasons*).

Fix

Use a websocket and update in real-time the firewall connection status.

* unresponsiveness, logs flooded by GET /api/firewall requests, unnecessary traffic...