The Splatable impl for Mgr used to fall back to the hovered item via
Tab::selected_or_hovered_urls() when the selection was empty, the same
convention used elsewhere (yank, copy, remove, open). PR #4108 rewrote
this into MgrSnap/TabSnap but only ported the plain selected list,
dropping the hovered fallback. This made %s and %d expand to nothing
when hovering a file without an explicit selection.
Restore the fallback in MgrSnap::selected(), which both %s/%S and
%d/%D read from.
Resolves#4132
