chatgui: add shutdown diagnostics — ~DbManager, ~SoundManager, g_shutdown_sentinel

Three diagnostic points covering the gap between ~MainWindow done and CRT exit:
- DbManager: destroyed — confirms local dtors started after ~MainWindow
- SoundManager: static destructor entered — confirms static destruction phase
- main: static destructors done (g_shutdown_sentinel dtor) — confirms ALL statics done

If process hangs, we know exactly which phase failed.

tools/chatgui/src/sound_manager.cpp

@@ -10,7 +10,10 @@ SoundManager* SoundManager::instance() {
     return &s;
 }
 
-SoundManager::~SoundManager() { shutdown(); }
+SoundManager::~SoundManager() {
+    DEBUG_INFO(DEBUG_CATEGORY_DEBUG, "SoundManager: static destructor entered");
+    shutdown();
+}
 
 bool SoundManager::init() {
     if (m_initialized) return true;