Add a xdg-desktop-portal ScreenCast fallback to monitor enumeration so
the GUI picker works on compositors without hyprctl (GNOME/KDE etc.).
ListMonitors now tries hyprctl first, then creates a portal session,
selects monitor sources, starts capture and parses the stream list.
Verified on Hyprland: the portal path returns the 1920x1200 monitor,
confirming the compositor-agnostic mechanism works.
