fix: skip JACK client deactivate() if server is already dead

jack.Client.deactivate() is a synchronous protocol call that hangs
indefinitely on a broken server socket. In the audio profile change
handler, stop_jack() kills the jackd process first, then tries
jack_client.stop() which calls deactivate() on the now-dead socket.

Fix: check if jackd is still running via pidof before calling
deactivate(). If the server is already gone, skip directly to
close() which just closes the local file descriptor (safe and fast).

This was causing the POST /api/audio/profile endpoint to hang for
30+ seconds on every buffer change.

src/system/audio.py

@@ -913,8 +913,12 @@ class JackAudioClient:
         """Deactivate and close the JACK client."""
         if not self._active or self._client is None:
             return
+        # Skip deactivate() if JACK server is already dead — it hangs
+        # on a broken server socket (Broken pipe).
         try:
-            self._client.deactivate()
+            import subprocess
+            if subprocess.run(["pidof", "jackd"], capture_output=True).returncode == 0:
+                self._client.deactivate()
         except Exception as exc:
             logger.warning("JACK deactivate error: %s", exc)
         try: