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/asturian_locale_test.dart

@@ -0,0 +1,73 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_localizations/flutter_localizations.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:tane/app.dart';
+import 'package:tane/i18n/strings.g.dart';
+import 'package:tane/services/locale_store.dart';
+import 'package:tane/ui/settings_screen.dart';
+
+import '../support/test_support.dart';
+
+/// Asturian (`ast`) is a first-class language, but Flutter ships no
+/// Material/Cupertino localizations for it. The app carries the honest `ast`
+/// locale for its own strings and borrows Spanish for the framework chrome
+/// (see [materialLocaleFor]). These tests pin both halves of that contract.
+void main() {
+  testWidgets('picking Asturian switches app text to Asturian, keeps Material '
+      'chrome resolvable in Spanish, and persists the choice', (tester) async {
+    LocaleSettings.setLocaleSync(AppLocale.en);
+    final store = LocaleStore(InMemorySecretStore());
+    late Locale materialLocale;
+
+    await tester.pumpWidget(
+      TranslationProvider(
+        child: Builder(
+          builder: (context) {
+            final appLocale = TranslationProvider.of(context).flutterLocale;
+            return MaterialApp(
+              // Mirrors app.dart: the framework localizations follow the mapped
+              // locale, the app's own strings follow slang.
+              locale: materialLocaleFor(appLocale),
+              supportedLocales: AppLocaleUtils.supportedLocales,
+              localizationsDelegates: const [
+                GlobalMaterialLocalizations.delegate,
+                GlobalWidgetsLocalizations.delegate,
+                GlobalCupertinoLocalizations.delegate,
+              ],
+              home: Builder(
+                builder: (context) {
+                  // Resolving these must never throw for the Asturian locale.
+                  materialLocale = Localizations.localeOf(context);
+                  MaterialLocalizations.of(context);
+                  return SettingsScreen(localeStore: store);
+                },
+              ),
+            );
+          },
+        ),
+      ),
+    );
+
+    // Starts in English.
+    expect(find.text('Language'), findsOneWidget);
+    expect(materialLocale.languageCode, 'en');
+
+    await tester.tap(find.text('Asturianu'));
+    await tester.pumpAndSettle();
+
+    // App strings are now Asturian…
+    expect(find.text('Llingua'), findsOneWidget);
+    // …the framework chrome fell back to Spanish (not the unsupported `ast`)…
+    expect(materialLocale.languageCode, 'es');
+    // …and the choice was remembered for next launch.
+    expect(await store.saved(), AppLocale.ast);
+  });
+
+  test('materialLocaleFor maps Asturian to Spanish and passes others through',
+      () {
+    expect(materialLocaleFor(const Locale('ast')), const Locale('es'));
+    expect(materialLocaleFor(const Locale('es')), const Locale('es'));
+    expect(materialLocaleFor(const Locale('en')), const Locale('en'));
+    expect(materialLocaleFor(const Locale('pt')), const Locale('pt'));
+  });
+}

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);
+}