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/bootstrap.dart

@@ -110,7 +110,9 @@ class BootstrapSplash extends StatelessWidget {
     return MaterialApp(
       debugShowCheckedModeBanner: false,
       theme: buildTaneTheme(),
-      locale: TranslationProvider.of(context).flutterLocale,
+      // Asturian (`ast`) has no bundled Material/Cupertino localizations, so map
+      // it to Spanish for the framework chrome — same as the main app (app.dart).
+      locale: materialLocaleFor(TranslationProvider.of(context).flutterLocale),
       supportedLocales: AppLocaleUtils.supportedLocales,
       localizationsDelegates: const [
         GlobalMaterialLocalizations.delegate,