feat(nixos-95): extract DE themining into nixosModule

the desktop environment can be included via `self.nixosModules.nixos95`

this will expose two options (for now):
nixos95.enable -> enable the desktop environment
nixos95.wallpaper -> path to the wallpaper to use

currently the setup is closy coupled to this configuration,
which will change in the future, so it can be used with any configuration

nixos95/core.nix

@@ -0,0 +1,22 @@
+{ config, lib, pkgs, ... }: let
+  cfg = config.nixos95;
+in lib.mkIf cfg.enable {
+
+  services.xserver = {
+    enable = true;
+    desktopManager.xfce.enable = true;
+    xkb = {
+      layout = "de";
+      variant = "";
+    };
+  };
+  services.xserver.displayManager.lightdm.enable = false;
+
+  services.displayManager.sddm = {
+    enable = true;
+    package = pkgs.qt6Packages.sddm;
+  };
+
+  xdg.portal.extraPortals = [ pkgs.xdg-desktop-portal-gtk ];
+
+}