Add nixpad configuration

The initial step of grouping lemp11 and t490 configuration, inspired by
https://github.com/mkellyxp/nixbook.

modules/nixos/default.nix

@@ -4,5 +4,6 @@
     ./core
     ./desktop
     ./homelab
+    ./nixpad
   ];
 }

modules/nixos/nixpad/default.nix

@@ -0,0 +1,37 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+
+let
+  cfg = config.nixpad;
+in
+{
+  options.nixpad = {
+    enable = lib.mkEnableOption "Enable nixpad configuration";
+  };
+
+  config = lib.mkIf cfg.enable {
+    services.xserver = {
+      enable = true;
+
+      desktopManager.cinnamon.enable = true;
+      displayManager.lightdm.enable = true;
+    };
+
+    programs.firefox.enable = true;
+
+    environment.systemPackages = with pkgs; [
+      git
+      libreoffice
+    ];
+
+    nix.gc = {
+      automatic = true;
+      dates = "Mon 4:00";
+      options = "--delete-older-than 30d";
+    };
+  };
+}