refactor config with better structure

flake.nix

@@ -27,6 +27,8 @@
       ...
     }:
     {
+      formatter.x86_64-linux = nixpkgs.legacyPackages.x86_64-linux.nixfmt-tree;
+
       nixosConfigurations.prouk = nixpkgs.lib.nixosSystem {
         system = "x86_64-linux";
         specialArgs = { inherit inputs; };