chatgui: add sound playback via miniaudio (MP3 notification sounds)

- SoundManager singleton wraps ma_engine + ma_sound (non-blocking, background thread)
- MP3 files embedded in QRC, decoded to PCM at startup with MA_SOUND_FLAG_DECODE
- miniaudio.h (MIT) in lib/, IMPLEMENTATION in separate miniaudio_impl.c
- Sound settings page in SettingsDialog: enable/disable, volume slider (0-100%)
- Config keys [gui] sound_enabled, sound_volume
- Incoming messages trigger msg_incoming.mp3 sound

tools/chatgui/transport/node_config.cpp

@@ -36,7 +36,8 @@ static const QSet<QString> CONTROL_KEYS = {
 };
 
 static const QSet<QString> GUI_KEYS = {
-    "db_path", "debug_file", "debug_level", "debug_categories"
+    "db_path", "debug_file", "debug_level", "debug_categories",
+    "sound_enabled", "sound_volume"
 };
 
 static const QSet<QString> ALLOWED_KEYS_KEYS = {