refactor config with better structure

nixos/desktop/niri.nix

@@ -0,0 +1,8 @@
+{ pkgs, inputs, ... }:
+{
+  programs.niri.enable = true;
+
+  environment.systemPackages = [
+    inputs.xwayland-satellite.packages.${pkgs.stdenv.hostPlatform.system}.default
+  ];
+}