refactor with better structure

nixos/desktop/sddm.nix

@@ -1,4 +1,9 @@
-{ pkgs, lib, config, ... }:
+{
+  pkgs,
+  lib,
+  config,
+  ...
+}:
 let
   # Same wallpaper niri displays (see home/dotfiles.nix).
   wallpaper = ../../assets/wallpaper.png;