Detect and report NATS disconnection

When a logged-in session loses its backend link, ORE Studio should notice promptly and tell the user, with a way to reconnect. Today it does not: if the NATS server is stopped, or the application services behind it are killed, the client carries on as though still connected — the status bar stays in its connected state and the user only discovers the problem when an action silently fails or hangs. This story makes the client detect the loss and surface a clear connection lost state.

• During an active session, the client detects loss of the NATS connection within a few seconds of it dropping (server stopped, network gone).
• It also detects the case where NATS is up but the application services stop responding — via heartbeat or request timeout — not only a raw socket drop.
• On detection the status bar moves to a clear connection lost state, visually distinct from the connected state, optionally with a banner.
• The user can reconnect without restarting the client; the saved connection details are reused (only re-authentication if required).
• When the backend returns, reconnecting restores the connected state and re-enables the workspace.
• The user manual's "Connection status and reconnection" section is updated to match the implemented behaviour (and the connection_lost_banner screenshot is captured).

• Automatic, silent background reconnection with no user action (possible follow-up).
• Queueing or replaying operations attempted while disconnected.