Click-and-drag reordering of favourites bar items in convergence
mode. The existing touch-based press-and-hold drag is kept for
mobile; in convergence mode, press-and-hold opens the context
menu only.
DelegateTouchArea owns the exclusive mouse grab at the C++ level,
so drag detection (threshold crossing + delta signals) is added
there rather than using a QML DragHandler. Displaced items
animate into their new positions while the dragged item follows
the cursor.
Expose FavouritesModel::moveEntry as Q_INVOKABLE so QML can
persist the reorder.