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/test/services/locale_store_test.dart

@@ -0,0 +1,54 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:tane/i18n/strings.g.dart';
+import 'package:tane/security/secret_store.dart';
+import 'package:tane/services/locale_store.dart';
+
+/// In-memory [SecretStore] for tests.
+class FakeSecretStore implements SecretStore {
+  final Map<String, String> _data = {};
+  @override
+  Future<String?> read(String key) async => _data[key];
+  @override
+  Future<void> write(String key, String value) async => _data[key] = value;
+}
+
+void main() {
+  // setLocale/useDeviceLocale touch the Flutter locale, which needs a binding.
+  TestWidgetsFlutterBinding.ensureInitialized();
+
+  late FakeSecretStore secrets;
+  late LocaleStore store;
+
+  setUp(() {
+    secrets = FakeSecretStore();
+    store = LocaleStore(secrets);
+  });
+
+  test('no choice saved yet → follows the device (null)', () async {
+    expect(await store.saved(), isNull);
+  });
+
+  test('picking Asturian persists it and applies it', () async {
+    await store.setLocale(AppLocale.ast);
+
+    expect(await store.saved(), AppLocale.ast);
+    expect(LocaleSettings.currentLocale, AppLocale.ast);
+    // Asturian is modelled as its real `ast` code so the schema stays honest;
+    // Material chrome borrows Spanish elsewhere (see app.dart).
+    expect(AppLocale.ast.languageCode, 'ast');
+  });
+
+  test('choosing "system language" forgets the explicit choice', () async {
+    await store.setLocale(AppLocale.es);
+    expect(await store.saved(), AppLocale.es);
+
+    await store.useDeviceLocale();
+    expect(await store.saved(), isNull);
+  });
+
+  test('an unknown stored tag reads as "follow the device", never throws',
+      () async {
+    await secrets.write('tane.locale', 'zz-ZZ-removed-in-a-later-build');
+    expect(await store.saved(), isNull);
+  });
+}