fix(i18n): map Asturian to Spanish chrome in boot splash

The pre-init BootstrapSplash passed the raw device locale to its
MaterialApp, so booting under an Asturian (ast) device locale threw
the 'locale not supported by localization delegates' warning — the
Material/Cupertino delegates ship no ast. The main app already routes
ast to Spanish framework chrome via materialLocaleFor; apply the same
mapping in the splash. Drop @visibleForTesting since it's now shared
production code.

apps/app_seeds/lib/app.dart

@@ -256,7 +256,6 @@ class TaneApp extends StatelessWidget {
 /// localizations can serve. Asturian (`ast`) has none, so it borrows Spanish
 /// (`es`) for the framework chrome; the app's own strings stay Asturian (they
 /// come from slang, not from `Localizations`). All other locales pass through.
-@visibleForTesting
 Locale materialLocaleFor(Locale locale) =>
     locale.languageCode == 'ast' ? const Locale('es') : locale;