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/ui/backup_section_test.dart

@@ -14,6 +14,7 @@ import 'package:tane/services/auto_backup_store.dart';
 import 'package:tane/services/export_import_service.dart';
 import 'package:tane/services/file_service.dart';
 import 'package:tane/services/recovery_sheet_service.dart';
+import 'package:tane/app.dart';
 import 'package:tane/ui/backup_section.dart';
 import 'package:tane/ui/settings_screen.dart';
 
@@ -254,6 +255,41 @@ void main() {
     expect(spy.calls, 1);
     expect(find.text('Copy saved'), findsOneWidget);
   });
+
+  testWidgets('the last-backup date renders under Asturian without an '
+      'invalid-locale crash', (tester) async {
+    // Asturian (`ast`) has no `intl` date symbols; formatting the "last copy"
+    // date used to throw "Invalid locale ast". The framework locale is mapped
+    // to Spanish, so the date must format through that instead.
+    LocaleSettings.setLocaleSync(AppLocale.ast);
+    addTearDown(() => LocaleSettings.setLocaleSync(AppLocale.en));
+
+    await tester.pumpWidget(
+      TranslationProvider(
+        child: MaterialApp(
+          locale: materialLocaleFor(const Locale('ast')),
+          supportedLocales: AppLocaleUtils.supportedLocales,
+          localizationsDelegates: const [
+            GlobalMaterialLocalizations.delegate,
+            GlobalWidgetsLocalizations.delegate,
+            GlobalCupertinoLocalizations.delegate,
+          ],
+          home: Scaffold(
+            body: BackupSection(
+              service: service,
+              sheet: sheet,
+              autoBackup: _DatedAutoBackup(db),
+            ),
+          ),
+        ),
+      ),
+    );
+    await tester.pump(); // resolve the "last copy" future
+
+    // The Asturian tile (and its Spanish-formatted date subtitle) built fine.
+    expect(find.text('Copies automátiques'), findsOneWidget);
+    expect(tester.takeException(), isNull);
+  });
 }
 
 /// Counts explicit "back up now" taps without touching the disk.
@@ -277,3 +313,21 @@ class _SpyAutoBackup extends AutoBackupService {
     return true;
   }
 }
+
+/// Reports a fixed "last copy" date so the auto-backup subtitle formats a real
+/// date — the path that crashed under Asturian.
+class _DatedAutoBackup extends AutoBackupService {
+  _DatedAutoBackup(AppDatabase db)
+    : super(
+        exporter: ExportImportService(
+          repository: newTestRepository(db),
+          files: FakeFileService(),
+          keys: SecureKeyStore(store: InMemorySecretStore()),
+        ),
+        store: AutoBackupStore(InMemorySecretStore()),
+        directory: () async => throw UnimplementedError(),
+      );
+
+  @override
+  Future<DateTime?> lastBackupAt() async => DateTime.utc(2026, 3, 14);
+}