feat: portal-based monitor enumeration fallback

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.

internal/capture/monitors.go

@@ -29,12 +29,16 @@ type Monitor struct {
 // ErrNoMonitors is returned when monitor enumeration is unsupported or fails.
 var ErrNoMonitors = errors.New("capture: monitor enumeration unavailable on this compositor")
 
-// ListMonitors returns the available monitors for the GUI picker.
+// ListMonitors returns the available monitors for the GUI picker. It prefers
+// the Hyprland IPC when available, falling back to the xdg-desktop-portal
+// ScreenCast API for other compositors.
 func ListMonitors() ([]Monitor, error) {
 	if hyprctlAvailable() {
-		return hyprctlMonitors()
+		if mons, err := hyprctlMonitors(); err == nil {
+			return mons, nil
+		}
 	}
-	return nil, ErrNoMonitors
+	return portalMonitors()
 }
 
 // hyprctlAvailable reports whether the Hyprland monitor command exists.