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/src/settingsdialog.cpp

@@ -4,6 +4,7 @@
 #include "databasesettingspage.h"
 #include "statuspage.h"
 #include "nodespage.h"
+#include "soundsettingspage.h"
 #include "../transport/gui_bridge.h"
 #include "../../lib/mem.h"
 #include "../db/db_manager.h"
@@ -44,6 +45,7 @@ SettingsDialog::SettingsDialog(const QString& configPath, DbManager* db, QWidget
     m_categoryList->addItem("Database");
     m_categoryList->addItem("Status");
     m_categoryList->addItem("Nodes");
+    m_categoryList->addItem("Sound");
     mainLayout->addWidget(m_categoryList);
 
     // --- Right panel: stacked pages ---
@@ -83,6 +85,9 @@ SettingsDialog::SettingsDialog(const QString& configPath, DbManager* db, QWidget
     // Nodes page
     m_nodesPage = new NodesPage(db, m_pages);
     m_pages->addWidget(m_nodesPage);
+
+    m_soundPage = new SoundSettingsPage(m_pages);
+    m_pages->addWidget(m_soundPage);
     rightLayout->addWidget(m_pages, 1);
 
     // --- Bottom buttons ---
@@ -119,6 +124,7 @@ SettingsDialog::SettingsDialog(const QString& configPath, DbManager* db, QWidget
     // Load current values
     loadProfileFromConfig(m_configPath);
     m_networkPage->loadFromConfig(m_configPath);
+    m_soundPage->loadFromConfig(m_configPath);
     m_categoryList->setCurrentRow(0);
 }
 
@@ -142,6 +148,7 @@ void SettingsDialog::onCategoryChanged(int row) {
     if (row == 2) m_databasePage->refreshStatus();
     if (row == 3) m_statusPage->refreshStatus();
     if (row == 4) m_nodesPage->refreshNodes();
+    if (row == 5) m_soundPage->loadFromConfig(m_configPath);
 }
 
 void SettingsDialog::onSave() {
@@ -181,5 +188,6 @@ void SettingsDialog::onSave() {
     f.close();
 
     m_networkPage->saveToConfig(m_configPath);
+    m_soundPage->saveToConfig(m_configPath);
     accept();
 }