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.
Enumerate monitors via hyprctl (Hyprland) and present them as a
dropdown in the settings window instead of a raw index. The picker is
enabled only for the screen source and persists the selected monitor as
stream_index. Falls back to a single indexed option when enumeration is
unavailable (non-Hyprland).
petere