[client] audio: make the requested audio device period size configurable

This adds a new `audio:periodSize` option which defaults to 2048 frames.
For PipeWire, this controls the `PIPEWIRE_LATENCY` value. For PulseAudio,
the controls the target buffer length (`tlength`) value.

client/src/main.h

@@ -199,6 +199,8 @@ struct AppParams
   bool              autoCapture;
   bool              captureInputOnly;
   bool              showCursorDot;
+
+  int               audioPeriodSize;
 };
 
 struct CBRequest