chatgui: track read position via debounced scroll, only fully visible msgs are read

- Debounce timer (300ms) advances lastReadMsgId only when message is fully in viewport
- Unread badge no longer resets on channel select — only when actual scroll reads msgs
- saveChannelState() uses tracked m_lastReadMsgId instead of viewport bottom
- getChannels() reads lastReadMsgId from ui_state (was reading unused channels column)

tools/chatgui/src/mainwindow.cpp

@@ -212,7 +212,6 @@ void MainWindow::setupConnects() {
             this, [this](const QString& channelId) {
                 QSettings().setValue("last_channel_id", channelId);
                 m_currentChannelId = channelId;
-                m_channelList->setChannelUnreadCount(channelId, 0);
                 m_messageList->loadChannel(channelId);
                 m_accountList->setChannel(channelId);
                 QByteArray cidBA = channelId.toUtf8();
@@ -223,6 +222,13 @@ void MainWindow::setupConnects() {
     connect(m_channelList, &ChannelList::joinRequested,       this, &MainWindow::showJoinDialog);
     connect(m_channelList, &ChannelList::createGroupRequested, this, &MainWindow::onCreateGroupRequested);
     connect(m_channelList, &ChannelList::settingsRequested,   this, &MainWindow::showSettings);
+    connect(m_messageList, &MessageList::readPositionChanged,
+            this, [this](const QString& channelId, qint64 lastReadMsgId) {
+                Q_UNUSED(lastReadMsgId);
+                if (m_channelList && m_db)
+                    m_channelList->setChannelUnreadCount(channelId,
+                        m_db->getUnreadCount(channelId, lastReadMsgId));
+            });
 }
 
 void MainWindow::setupTray() {