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/db/db_manager.cpp

@@ -1,5 +1,6 @@
 #include "db_manager.h"
 #include "../src/debug_ui.h"
+#include "../../lib/debug_config.h"
 #include <openssl/sha.h>
 #include <openssl/evp.h>
 #include <cstring>
@@ -78,6 +79,7 @@ DbManager::~DbManager() {
     for (auto it = m_cursors.begin(); it != m_cursors.end(); ++it)
         sqlite3_finalize(it.value());
     m_cursors.clear();
+    DEBUG_INFO(DEBUG_CATEGORY_DEBUG, "DbManager: destroyed");
 }
 
 sqlite3_stmt* DbManager::prepareOrNull(const char* sql) const {