fix: replace removed programs.light with brightnessctl

programs.light was removed from nixpkgs (upstream unmaintained).
brightnessctl is the recommended replacement for Sway/Wayland.

roles/desktop/wayland.nix

@@ -11,9 +11,8 @@ in
       enable = true;
       wrapperFeatures.gtk = true;
     };
-    programs.light.enable = true;
-
     environment.systemPackages = with pkgs; [
+      brightnessctl
       grim
       slurp
       wl-clipboard