chatgui: per-event sound settings with DB persistence

- Sound files loaded from filesystem (sound_dir in INI config, absolute or relative)
- Per-event enable/disable + volume stored in DB (ui_state table)
- SoundManager: setEvent(), effective volume = master * per-event
- SoundSettingsPage UI: 4 event rows (checkbox + slider)
- chat_core_save_ui_state() for async DB writes via uasync thread
- DbManager::getUiStateInt() for reading settings from GUI thread
- Default sounds copied to sounds/ dir next to DB, config entries written

tools/chatgui/transport/node_config.cpp

@@ -37,7 +37,8 @@ static const QSet<QString> CONTROL_KEYS = {
 
 static const QSet<QString> GUI_KEYS = {
     "db_path", "debug_file", "debug_level", "debug_categories",
-    "sound_enabled", "sound_volume"
+    "sound_dir", "sound_msg_incoming", "sound_msg_outgoing",
+    "sound_connect", "sound_disconnect"
 };
 
 static const QSet<QString> ALLOWED_KEYS_KEYS = {