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/main.cpp

@@ -10,6 +10,7 @@
 #include "../db/db_manager.h"
 #include "../transport/node_config.h"
 #include "../transport/utun_node.h"
+#include "../../lib/debug_config.h"
 
 #ifdef _WIN32
 static UtunNode* g_ctrl_node = nullptr;
@@ -23,6 +24,12 @@ static BOOL WINAPI console_ctrl_handler(DWORD dwCtrlType) {
 }
 #endif
 
+static struct ShutdownSentinel {
+    ~ShutdownSentinel() {
+        DEBUG_INFO(DEBUG_CATEGORY_DEBUG, "main: static destructors done");
+    }
+} g_shutdown_sentinel;
+
 int main(int argc, char *argv[]) {
     QApplication app(argc, argv);
     app.setApplicationName("Chat");
@@ -98,6 +105,6 @@ int main(int argc, char *argv[]) {
 #endif
 
     int ret = app.exec();
-    qDebug("main: app.exec() returned %d", ret);
+    DEBUG_INFO(DEBUG_CATEGORY_DEBUG, "main: app.exec() returned %d", ret);
     return ret;
 }