fix: handle monitor hotplug for greeter windows (v0.8.2)

Greeter windows were only created at startup. Hotplugged monitors (e.g.
HDMI reconnect) would show no UI. Connect to the monitor ListModel's
items-changed signal to create greeter windows for newly added monitors.

Aligned with moonlock's hotplug fix using the same pattern adapted for
gtk4-layer-shell (ListModel) instead of session-lock (connect_monitor).

Cargo.lock

@@ -575,7 +575,7 @@ dependencies = [
 
 [[package]]
 name = "moongreet"
-version = "0.7.4"
+version = "0.8.0"
 dependencies = [
  "gdk-pixbuf",
  "gdk4",