feat(sddm): custom background

systems/nel-desktop/configuration.nix

···
-{ pkgs, ... }: {
+{ pkgs, config, ... }: {
   imports = [
     ../../modules/nixos
     ./hardware-configuration.nix
···
 
   services.displayManager.sddm.enable = true;
   services.desktopManager.plasma6.enable = true;
+  environment.systemPackages = [
+    (pkgs.writeTextDir "share/sddm/themes/${config.services.displayManager.sddm.theme}/theme.conf.user" ''
+      [General]
+      background=${./transcoded-wallpaper.jpeg}
+    '')
+  ];
 
   services.pulseaudio.enable = false;
   security.rtkit.enable = true;