Fix liveview browser freeze from inifinite loop

[mcmah309]

Waiting for a Rust message could trap JavaScript in an infinite loop. The previous recv() waited 50 ms, then executed:

  while (true) {
      let __data = _message_queue.shift();
      if (__data) {
          __msg = __data;
          break;
      }
  }

If the queue was empty, this loop never yielded. The browser could no longer process clicks, render updates, or execute the WebSocket handler that delivers the awaited message. The receiver blocked the very mechanism needed to unblock it. The timeout only delayed entering the loop; it did not make the loop asynchronous.

[nikuda]

Same issue addressed here #5848