nix flake update, add overlay and nixosModule

flake.lock

@@ -2,11 +2,11 @@
   "nodes": {
     "nixpkgs": {
       "locked": {
-        "lastModified": 1754292888,
-        "narHash": "sha256-1ziydHSiDuSnaiPzCQh1mRFBsM2d2yRX9I+5OPGEmIE=",
+        "lastModified": 1757545623,
+        "narHash": "sha256-mCxPABZ6jRjUQx3bPP4vjA68ETbPLNz9V2pk9tO7pRQ=",
         "owner": "nixos",
         "repo": "nixpkgs",
-        "rev": "ce01daebf8489ba97bd1609d185ea276efdeb121",
+        "rev": "8cd5ce828d5d1d16feff37340171a98fc3bf6526",
         "type": "github"
       },
       "original": {

flake.nix

@@ -25,10 +25,18 @@
     {
       packages = forAllSystems (import ./packages.nix);
 
+      overlays.default = final: prev: {
+        zerforschen-plus-content = self.packages."${prev.system}".zerforschen-plus-content;
+      };
+
+      nixosModules.default = {
+        nixpkgs.overlays = [ self.overlays.default ];
+      };
+
       devShells = forAllSystems (
         { pkgs, system, ... }:
         {
-          default = pkgs.mkShellNoCC rec {
+          default = pkgs.mkShellNoCC {
             inputsFrom = [ self.packages.${system}.default ];
 
             shellHook = ''
@@ -39,6 +47,6 @@
         }
       );
 
-      formatter = forAllSystems ({ pkgs, ... }: pkgs.nixfmt-rfc-style);
+      formatter = forAllSystems ({ pkgs, ... }: pkgs.nixfmt-tree);
     };
 }