Add NixOS support with configuration module

- Add nixos/kawa.nix module for NixOS integration
- Add flake.nix for Nix flakes support
- Update setup script to detect and handle NixOS
- Include NATS, Syncthing, and Tailscale configuration
- Add parameters for Headscale connection

flake.nix

@@ -0,0 +1,30 @@
+{
+  description = "KAWA OS - NixOS Configuration";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+  };
+
+  outputs = { self, nixpkgs, ... }@inputs: {
+    nixosModules = {
+      default = import ./nixos;
+      kawa = import ./nixos/kawa.nix;
+    };
+    
+    nixosConfigurations = {
+      # Exemple de configuration
+      kawa-node = nixpkgs.lib.nixosSystem {
+        system = "x86_64-linux";
+        modules = [
+          ./nixos
+          {
+            services.kawa = {
+              enable = true;
+              hostname = "kawa-node-01";
+            };
+          }
+        ];
+      };
+    };
+  };
+}