chatgui: rework shutdown — main thread owns cleanup, remove glШбаls

Changes:
- UtunNode::requestStop() — async stop from any thread (m_stop+wakeup)
- UtunNode::finalize() — cleanup in main thread (etcp_router, chat_sync, chat_core, utun_instance, uasync)
- UtunNode::stop() — requestStop + join + finalize (for tray Quit path)
- Worker runLoop: only exits poll loop on m_stop, then emits stopped — no cleanup
- stopped() handler: calls finalize() in main thread, then qApp->quit()
- Tray Quit: calls requestStop(), stopped handler does finalize+quit
- Ctrl-C (Windows): SetConsoleCtrlHandler calls requestStop() via g_ctrl_node
- Ctrl-C (Linux): signal_handler calls requestStop() via g_currentNode
- Removed: g_shutdown, g_signal_ua, utun_node_signal_shutdown globals

tools/chatgui/src/mainwindow.cpp

@@ -120,10 +120,11 @@ void MainWindow::setupNode() {
     connect(m_node, &UtunNode::started, this, []() {
         qDebug("UtunNode started");
     });
-    connect(m_node, &UtunNode::stopped, this, []() {
+    connect(m_node, &UtunNode::stopped, this, [this]() {
         qDebug("UtunNode stopped");
+        if (m_node) m_node->finalize();
+        qApp->quit();
     });
-    connect(m_node, &UtunNode::stopped, qApp, &QApplication::quit);
 
     if (!m_cfgDebugFile.isEmpty())
         m_node->setDebugFile(m_cfgDebugFile);
@@ -255,7 +256,9 @@ void MainWindow::setupTray() {
     });
 
     QAction *quitAction = m_trayMenu->addAction("Quit");
-    connect(quitAction, &QAction::triggered, qApp, &QApplication::quit);
+    connect(quitAction, &QAction::triggered, this, [this]() {
+        if (m_node) m_node->requestStop();
+    });
 
     m_trayIcon->setContextMenu(m_trayMenu);