modularize DE-related system config

modules/desktop.nix

@@ -0,0 +1,68 @@
+{ config, pkgs, lib, ... }:
+
+{
+  # GNOME Desktop Environment
+  services.xserver.enable = true;
+  services.displayManager.gdm.enable = true;
+  services.desktopManager.gnome.enable = true;
+
+  # Keyboard layout
+  services.xserver.xkb = {
+    layout = "us";
+    options = "";
+  };
+
+  # Exclude unwanted GNOME default packages
+  environment.gnome.excludePackages = with pkgs; [
+    gnome-tour
+    gnome-console  # terminal (using Ghostty instead)
+    gnome-text-editor  # text editor (using Neovim instead)
+    gnome-connections  # remote desktop client
+    gnome-font-viewer  # font viewer
+    seahorse  # passwords and keys
+    baobab  # disk usage analyzer
+    gnome-disk-utility  # disks
+    gnome-logs  # logs viewer
+    gnome-system-monitor  # system monitor
+    decibels  # audio player
+    epiphany  # GNOME web browser
+    file-roller  # archive manager
+    geary     # GNOME email client
+    gnome-music
+    gnome-photos
+    gnome-maps
+    gnome-weather
+    gnome-contacts
+    gnome-clocks
+    gnome-calculator
+    gnome-calendar
+    gnome-characters
+    simple-scan
+    snapshot  # camera
+    totem     # video player
+    yelp      # help viewer
+  ];
+
+  # XDG portal for proper desktop integration
+  xdg.portal = {
+    enable = true;
+    extraPortals = with pkgs; [
+      xdg-desktop-portal-gnome
+      xdg-desktop-portal-gtk
+    ];
+  };
+
+  # Touchpad configuration
+  services.libinput = {
+    enable = true;
+    touchpad = {
+      naturalScrolling = true;
+      tapping = true;
+    };
+  };
+
+  # System packages for GNOME
+  environment.systemPackages = with pkgs; [
+    hicolor-icon-theme  # Fallback icon theme
+  ];
+}