fix(extension): suppress ERR_CONNECTION_REFUSED and harden WebSocket lifecycle

[huangsen365]

Problem

When the opencli daemon is not running, the Chrome extension's WebSocket reconnect loop produces noisy ERR_CONNECTION_REFUSED console errors every few seconds:

WebSocket connection to 'ws://localhost:19825/ext' failed: 
Error in connection establishment: net::ERR_CONNECTION_REFUSED

These errors:

• Cannot be caught via try/catch (Chrome logs them before JS gets control)
• Clutter the Service Worker console
• May confuse users into thinking something is broken

Additionally, several race conditions existed in the WebSocket lifecycle.

Solution

Probe before connect: Use fetch(HEAD) to check if the daemon is reachable before opening a WebSocket. A failed fetch is silent — no console noise. Only proceed with WebSocket when daemon is confirmed up.

Harden lifecycle:

• Add connecting guard flag to prevent concurrent connect() calls from overlapping alarm/event triggers
• Check WebSocket.CLOSING state to avoid creating a new socket while the old one is still shutting down
• Capture socket in local variable to prevent stale reference races between onclose/onerror callbacks
• Wrap socket.close() in try/catch in onerror (can throw during SW teardown)
• Only send command responses if the socket is still the active one and OPEN

Testing

• Daemon stopped → extension quietly retries with exponential backoff, no console errors
• Daemon started → extension connects within one backoff cycle
• Daemon restarted → extension reconnects cleanly after onclose fires