app: replace audio send polling with blocking queue.get

Deterministic wake on data arrival instead of 5ms sleep loop.
Reduces CPU wake-ups and eliminates up to 5ms of added latency.

src/tuimble/app.py

@@ -676,13 +676,11 @@ class TuimbleApp(App):
 
     @work(thread=True)
     def _audio_send_loop(self) -> None:
-        """Poll capture queue and send encoded frames to server."""
+        """Send captured audio frames to the server."""
         while self._client.connected:
-            frame = self._audio.get_capture_frame()
+            frame = self._audio.get_capture_frame(timeout=0.02)
             if frame is not None:
                 self._client.send_audio(frame)
-            else:
-                time.sleep(0.005)
 
     # -- channel tree --------------------------------------------------------