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

nixos/default.nix

@@ -0,0 +1,18 @@
+# KAWA OS - Configuration par défaut
+# Usage: import dans configuration.nix
+
+{ config, lib, pkgs, ... }:
+
+{
+  imports = [
+    ./kawa.nix
+  ];
+  
+  services.kawa = {
+    enable = true;
+    autoConnect = true;
+    enableNats = true;
+    enableSyncthing = true;
+    enableOllama = false; # Activer si GPU disponible
+  };
+}