Treat cua-driver's Linux `is_on_screen: null` as unknown instead of
off-screen, and skip GNOME Shell desktop/backdrop helper windows
(ding "Desktop Icons", @!x,y;BDHF) when selecting the default capture
target — they are targetable X11 windows but capture as empty.
Reconciled with the _NET_ACTIVE_WINDOW fallback from #58030: helper
windows are filtered out of the candidate pool first, then the tied
z-order active-window probe runs on the remaining real app windows.
Also falls back to the requested app name for _last_app when Linux
windows carry no app_name.
Salvaged from #54173 by @dnth.
