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

Configurations/Hosts/Default/configuration.nix

@@ -23,48 +23,24 @@ in {
     # Home Manager integration
     inputs.home-manager.nixosModules.home-manager
 
+    # nixos95
+    inputs.self.nixosModules.nixos95
+
   ];
   ################################################################
   # Display & Desktop Environment
   ################################################################
 
-  services.xserver = {
+  nixos95 = {
     enable = true;
-    desktopManager.xfce.enable = true;
-    xkb = {
-      layout = "de";
-      variant = "";
-    };
+    user = username;
   };
 
-  services.xserver.displayManager.lightdm.enable = false;
-
-  services.displayManager.sddm = {
-    enable = true;
-    package = pkgs.qt6Packages.sddm;
-  };
-
-  xdg.portal.extraPortals = [ pkgs.xdg-desktop-portal-gtk ];
-
   ################################################################
   # System Packages (XFCE & Utilities)
   ################################################################
 
   environment.systemPackages = with pkgs; [
-    xdg-desktop-portal-gtk
-
-    # XFCE Core
-    xfce.xfwm4
-    xfce.xfce4-panel
-    xfce.xfce4-session
-    xfce.xfce4-settings
-    xfce.thunar
-    xfce.mousepad
-    xfce.xfce4-terminal
-    xfce.xfce4-appfinder
-    xfce.xfce4-power-manager
-    xfce.xfce4-notifyd
-    xfce.xfce4-whiskermenu-plugin
 
     # Optional Extras
     xfce.gigolo
@@ -136,4 +112,4 @@ in {
   ################################################################
 
   system.stateVersion = "25.05";
-}
+}