refactor(nixos): default regional settings to neutral values

The public baseline hard-coded a German keymap and Europe/Berlin time
zone — personal values a consumer would have to override. Default them to
region-agnostic values instead (`us` keymap, `UTC`, `en_US` locale), all
`mkDefault`, so a private inventory sets the real regional values on top
without needing `mkForce`.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

modules/nixos.nix

@@ -41,14 +41,14 @@
     ];
   };
 
-  # Time zone. Template default — override per host/region.
-  time.timeZone = "Europe/Berlin";
-
-  # Internationalisation properties. Template defaults — override per region.
+  # Neutral template defaults for regional/localized settings. All are
+  # mkDefault: set the real values (time zone, keymap, locale categories) in your
+  # own config so this public baseline stays region-agnostic.
+  time.timeZone = lib.mkDefault "UTC";
   i18n.defaultLocale = lib.mkDefault "en_US.UTF-8";
   console = {
-    font = "Lat2-Terminus16";
-    keyMap = "de";
+    font = lib.mkDefault "Lat2-Terminus16";
+    keyMap = lib.mkDefault "us";
   };
 
   # Programs.