chatgui: add SIGINT/SIGTERM handler (as in utun.c), save scroll pos on every debounce

Signal handler via g_shutdown + uasync_wakeup ensures clean uasync poll exit.
updateReadPosition() now saves both lastReadMsgId and lastPosMsgId on every
scroll stop (300ms debounce), not just when read position changes.
connect(stopped, quit) triggers graceful Qt shutdown after uasync cleanup.

tools/chatgui/src/mainwindow.cpp

@@ -84,6 +84,8 @@ MainWindow::MainWindow(QWidget *parent, DbManager* db, const QString& cfgPath,
 }
 
 MainWindow::~MainWindow() {
+    DEBUG_DEBUG(DEBUG_CATEGORY_DEBUG, "MainWindow: ~MainWindow — saving channel state");
+    if (m_messageList) m_messageList->saveChannelState();
     DEBUG_DEBUG(DEBUG_CATEGORY_DEBUG, "MainWindow: ~MainWindow — stopping utun node");
     if (m_node) { m_node->stop(); delete m_node; m_node = nullptr; }
     DEBUG_DEBUG(DEBUG_CATEGORY_DEBUG, "MainWindow: ~MainWindow done");
@@ -120,6 +122,7 @@ void MainWindow::setupNode() {
     connect(m_node, &UtunNode::stopped, this, []() {
         qDebug("UtunNode stopped");
     });
+    connect(m_node, &UtunNode::stopped, qApp, &QApplication::quit);
 
     if (!m_cfgDebugFile.isEmpty())
         m_node->setDebugFile(m_cfgDebugFile);
@@ -230,11 +233,6 @@ void MainWindow::setupConnects() {
                     m_channelList->setChannelUnreadCount(channelId,
                         m_db->getUnreadCount(channelId, lastReadMsgId));
             });
-
-    connect(qApp, &QApplication::aboutToQuit, this, [this]() {
-        DEBUG_DEBUG(DEBUG_CATEGORY_DEBUG, "MainWindow: aboutToQuit — saving channel state");
-        if (m_messageList) m_messageList->saveChannelState();
-    });
 }
 
 void MainWindow::setupTray() {