feat: apply cursor theme via GtkSettings (v0.9.0)

GTK4 under greetd does not honour XCURSOR_THEME reliably, so the greeter
showed the wrong cursor over GTK widgets on some machines. Set
gtk-cursor-theme-name / -size on GtkSettings from new [appearance] config
fields (cursor-theme, cursor-size), mirroring the gtk-theme handling and
reusing is_valid_gtk_theme validation.

Salvaged from commit 29ce185 — tagged v0.9.0 on 2026-04-24 but never
pushed. The orphan branch's keyboard refactor (v0.10.0) is discarded,
superseded by the v0.8.7 single-window fix.

Cargo.lock

@@ -575,7 +575,7 @@ dependencies = [
 
 [[package]]
 name = "moongreet"
-version = "0.8.7"
+version = "0.9.0"
 dependencies = [
  "gdk-pixbuf",
  "gdk4",