refactor: optimize flake.nix and modularize networking config

Cleaner deploy.nodes generation, improved devShell experience, and centralized host IP definitions.

test/vm-test.nix

@@ -0,0 +1,29 @@
+{ self, nixpkgs, ... }:
+
+let
+  system = "x86_64-linux";
+  pkgs = nixpkgs.legacyPackages.${system};
+in
+pkgs.nixosTest {
+  name = "deploy-user-test";
+  nodes = {
+    machine = { ... }: {
+      imports = [
+        ../modules
+        ../users
+      ];
+      homelab.users.deploy.enable = true;
+      system.stateVersion = "24.11"; # Match the current nixpkgs version
+    };
+  };
+
+  testScript = ''
+    machine.wait_for_unit("multi-user.target")
+    # Verify user exists
+    machine.succeed("id deploy")
+    # Verify we can run nix-env as deploy via sudo
+    machine.succeed("sudo -u deploy -n nix-env --version")
+    # Verify switch-to-configuration is accessible (it's added to path by the module)
+    machine.succeed("whereis switch-to-configuration")
+  '';
+}