refactor(nixos): centralize universal defaults

configs/nixos/modules/default.nix

@@ -48,6 +48,10 @@ in
       "flakes"
     ];
 
+    system.stateVersion = "26.05";
+    time.timeZone = "America/New_York";
+    i18n.defaultLocale = "en_GB.UTF-8";
+
     users.users.${user.name} = {
       isNormalUser = true;
       description = user.description;

configs/nixos/modules/guest.nix

@@ -6,8 +6,6 @@ in
   options.modules.guest.enable = lib.mkEnableOption "nested-guest base configuration";
 
   config = lib.mkIf cfg.enable {
-    system.stateVersion = "26.05";
-
     modules.network.ssh.enable = lib.mkDefault true;
     modules.network.ssh.hostKeys.restore = lib.mkDefault false;
   };