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/ui/backup_section.dart

@@ -360,8 +360,11 @@ class _AutoBackupTileState extends State<_AutoBackupTile> {
         final subtitle = last == null
             ? t.backup.autoBackupNone
             : t.backup.autoBackupLast(
+                // Format via the Localizations locale, not the raw app locale:
+                // Asturian (`ast`) has no `intl` date symbols and would throw,
+                // but it's mapped to Spanish for the framework (see app.dart).
                 date: DateFormat.yMMMd(
-                  LocaleSettings.currentLocale.languageCode,
+                  Localizations.localeOf(context).languageCode,
                 ).format(last),
               );
         return ListTile(

apps/app_seeds/lib/ui/settings_screen.dart

@@ -1,23 +1,30 @@
 import 'package:flutter/material.dart';
 import 'package:go_router/go_router.dart';
 
+import '../di/injector.dart';
 import '../i18n/strings.g.dart';
 import '../services/export_import_service.dart';
+import '../services/locale_store.dart';
 import 'backup_section.dart';
 import 'theme.dart';
 
 /// Basic settings: app language, backup (export/import) and an "about"
-/// section. [exportImport] is injectable so widget tests can pass a fake;
-/// the app resolves it lazily from the service locator.
+/// section. [exportImport] and [localeStore] are injectable so widget tests can
+/// pass fakes; the app resolves them lazily from the service locator.
 class SettingsScreen extends StatelessWidget {
-  const SettingsScreen({this.exportImport, super.key});
+  const SettingsScreen({this.exportImport, this.localeStore, super.key});
 
   final ExportImportService? exportImport;
+  final LocaleStore? localeStore;
 
   @override
   Widget build(BuildContext context) {
     final t = context.t;
-    final current = TranslationProvider.of(context).flutterLocale.languageCode;
+    // Compare the full locale (not just the language code) so `es` and the
+    // Asturian `es-AST` — which share the `es` language — are told apart.
+    final current = TranslationProvider.of(context).locale;
+    void pick(AppLocale locale) =>
+        (localeStore ?? getIt<LocaleStore>()).setLocale(locale);
     return Scaffold(
       appBar: AppBar(title: Text(t.menu.settings)),
       body: ListView(
@@ -25,23 +32,28 @@ class SettingsScreen extends StatelessWidget {
           _SectionHeader(t.settings.language),
           _LanguageTile(
             label: t.settings.langEs,
-            selected: current == 'es',
-            onTap: () => LocaleSettings.setLocale(AppLocale.es),
+            selected: current == AppLocale.es,
+            onTap: () => pick(AppLocale.es),
+          ),
+          _LanguageTile(
+            label: t.settings.langAst,
+            selected: current == AppLocale.ast,
+            onTap: () => pick(AppLocale.ast),
           ),
           _LanguageTile(
             label: t.settings.langEn,
-            selected: current == 'en',
-            onTap: () => LocaleSettings.setLocale(AppLocale.en),
+            selected: current == AppLocale.en,
+            onTap: () => pick(AppLocale.en),
           ),
           _LanguageTile(
             label: t.settings.langPt,
-            selected: current == 'pt',
-            onTap: () => LocaleSettings.setLocale(AppLocale.pt),
+            selected: current == AppLocale.pt,
+            onTap: () => pick(AppLocale.pt),
           ),
           ListTile(
             leading: const Icon(Icons.smartphone_outlined),
             title: Text(t.settings.systemLanguage),
-            onTap: () => LocaleSettings.useDeviceLocale(),
+            onTap: () => (localeStore ?? getIt<LocaleStore>()).useDeviceLocale(),
           ),
           const Divider(),
           _SectionHeader(t.backup.title),