feat(i18n): add Asturian (asturianu) as a language

Adds Asturianu, translated in full (345 strings), plus the wiring to
make a minority language work end to end — the OS language picker is an
unreliable way to reach it, so the in-app choice must be explicit and
remembered.

- Model Asturian as the real `ast` locale (honest, international-by-design),
  not a fake `es-AST` region — slang rejects a 3-letter region anyway.
- Flutter ships no Material/Cupertino/intl localizations for `ast`, so map
  the framework chrome to Spanish (`materialLocaleFor`); the app's own text
  stays Asturian via slang. Fixes an "Invalid locale ast" crash when the
  auto-backup date was formatted with intl.
- Persist the picked language in the keystore (LocaleStore) and restore it
  at startup so it survives restarts and wins over the device locale.
- Settings: add the Asturianu tile; compare the full AppLocale so `es` and
  `ast` aren't both marked selected.

Tests: LocaleStore round-trip, locale switch + Spanish-chrome fallback,
and a regression for the intl date crash under Asturian.

apps/app_seeds/lib/main.dart

@@ -7,6 +7,7 @@ import 'di/injector.dart';
 import 'i18n/strings.g.dart';
 import 'services/auto_backup_service.dart';
 import 'services/coarse_location.dart';
+import 'services/locale_store.dart';
 import 'services/message_store.dart';
 import 'services/offer_outbox.dart';
 import 'services/onboarding_store.dart';
@@ -17,8 +18,13 @@ import 'services/social_settings.dart';
 
 Future<void> main() async {
   WidgetsFlutterBinding.ensureInitialized();
+  // Start from the device language, then let an explicit in-app choice win —
+  // it's the only reliable way to reach languages the OS picker doesn't offer
+  // (e.g. Asturian). Restored after DI, since the store lives in the keystore.
   LocaleSettings.useDeviceLocaleSync();
   await configureDependencies();
+  final savedLocale = await getIt<LocaleStore>().saved();
+  if (savedLocale != null) LocaleSettings.setLocaleSync(savedLocale);
   final onboarding = getIt<OnboardingStore>();
   runApp(
     TranslationProvider(