- Create CListCtrlEx class derived from CListCtrl
- Support right-click header menu to toggle column visibility
- Save column visibility settings to registry (list\{ConfigKey})
- Settings persist by column name, not index (order-independent)
- Skip empty column titles in context menu
- Update CClientListDlg to use CListCtrlEx
- Update m_CList_Online in 2015RemoteDlg to use CListCtrlEx