fractalart service: init, Generate wallpapers (#23231)

* Add a service for generating wallpapers
This service will generate a new wallpaper using the "FractalArt" haskell package.

nixos/modules/module-list.nix

@@ -639,6 +639,7 @@
   ./services/x11/display-managers/sddm.nix
   ./services/x11/display-managers/slim.nix
   ./services/x11/display-managers/xpra.nix
+  ./services/x11/fractalart.nix
   ./services/x11/hardware/libinput.nix
   ./services/x11/hardware/multitouch.nix
   ./services/x11/hardware/synaptics.nix

nixos/modules/services/x11/fractalart.nix

@@ -0,0 +1,36 @@
+{ config, lib, pkgs, ... }:
+with lib;
+let
+  cfg = config.services.fractalart;
+in {
+  options.services.fractalart = {
+    enable = mkOption {
+      type = types.bool;
+      default = false;
+      example = true;
+      description = "Enable FractalArt for generating colorful wallpapers on login";
+    };
+
+    width = mkOption {
+      type = types.nullOr types.int;
+      default = null;
+      example = 1920;
+      description = "Screen width";
+    };
+
+    height = mkOption {
+      type = types.nullOr types.int;
+      default = null;
+      example = 1080;
+      description = "Screen height";
+    };
+  };
+
+  config = mkIf cfg.enable {
+    environment.systemPackages = [ pkgs.haskellPackages.FractalArt ];
+    services.xserver.displayManager.sessionCommands =
+      "${pkgs.haskellPackages.FractalArt}/bin/FractalArt --no-bg -f .background-image"
+        + optionalString (cfg.width  != null) " -w ${toString cfg.width}"
+        + optionalString (cfg.height != null) " -h ${toString cfg.height}";
+  };
+}