chatgui: channel sorting by name or last message time with preview

- Add last_msg_at column to channels table
- Update last_msg_at on every message insert (send/receive)
- Parameterize getChannels() with ChannelSortMode enum (Alphabetical/Recent)
- Add getChannelLastMessage() for preview text in channel list
- Add sorting combo to Profile settings page (stored in ui_state)
- Reload channels on message receive when sort is recent

tools/chatgui/src/mainwindow.cpp

@@ -340,6 +340,7 @@ void MainWindow::showJoinDialog() {
 void MainWindow::showSettings() {
     SettingsDialog dlg(m_cfgPath, m_db, m_node, this);
     dlg.exec();
+    reloadChannels();
 }
 
 void MainWindow::onMessageReceived(const char* ch_id, int ch_id_len, uint64_t author) {
@@ -358,6 +359,8 @@ void MainWindow::onMessageReceived(const char* ch_id, int ch_id_len, uint64_t au
             m_channelList->setChannelUnreadCount(cid, unread);
         }
     }
+    if (m_db && m_db->getUiStateInt("channel_sort", 0) == 1)
+        reloadChannels();
 }
 
 void MainWindow::onMembersChanged(const char* ch_id) {