Fall back to REST polling when the WebSocket cannot connect

On some mobile networks/browsers the wss:// upgrade is blocked even though the
page loads (the server side is verified working: a browser-style handshake gets
101). After three socket failures the header now shows "polling (5s)" instead
of a misleading perpetual "reconnecting", since the 5s REST poll keeps all data
correct. The socket error handler also surfaces the browser's message.

frontend/src/api.js

@@ -58,7 +58,12 @@ export function openSocket({ onMessage, onOpen, onClose, onError }) {
       onError?.(event.code, event.reason)
       schedule()
     }
-    socket.onerror = () => socket.close()
+    socket.onerror = (event) => {
+      // The browser hides details here; surface what it does give us.
+      const detail = event?.message || 'connection error'
+      onError?.(0, detail)
+      socket.close()
+    }
   }
   connect()