refactor(i18n): make defaultLocale overridable via mkDefault

Template hard-set `en_US.UTF-8`, so a consumer setting `i18n.defaultLocale`
in the private inventory hit a conflicting-definition eval error. Wrap the
template default in `lib.mkDefault` per the overrides-are-options contract.

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

modules/nixos.nix

@@ -45,7 +45,7 @@
   time.timeZone = "Europe/Berlin";
 
   # Internationalisation properties. Template defaults — override per region.
-  i18n.defaultLocale = "en_US.UTF-8";
+  i18n.defaultLocale = lib.mkDefault "en_US.UTF-8";
   console = {
     font = "Lat2-Terminus16";
     keyMap = "de";