fix: pause support + old message skip + stopCurrentPcm

- SoundManager: added stopCurrentPcm() for pause/stop of PCM playback
- messagedelegate: call stopCurrentPcm on pause, new play, and timer complete
- messagelist: skip old double-nested JSON messages (raw starts with '{')
- sound_manager.h: void* for ma_audio_buffer to avoid header dependency

tools/chatgui/src/audiorecorder.cpp

@@ -1,6 +1,6 @@
 #include "audiorecorder.h"
-#include "sound_manager.h"
 #include "miniaudio.h"
+#include "sound_manager.h"
 
 #include <QDebug>
 #include <cstring>