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/services/locale_store.dart

@@ -0,0 +1,45 @@
+import '../i18n/strings.g.dart';
+import '../security/secret_store.dart';
+
+/// Remembers the language the user picked in Settings so it survives restarts.
+/// An empty/absent value means "follow the device language". Backed by the OS
+/// keystore (via [SecretStore]), like the rest of the app's small preferences —
+/// honouring "no plaintext at rest".
+///
+/// Why persist at all: the device language picker is an unreliable way to reach
+/// minority languages (Android often doesn't offer Asturian, and slang would
+/// fall back to the device default on every launch), so the choice made in-app
+/// must be remembered explicitly. Asturian itself is modelled as the `es-AST`
+/// locale (Spanish language + `AST` region), not `ast`, so Flutter's
+/// Material/Cupertino localizations — which don't ship `ast` — resolve it
+/// through Spanish instead of throwing. See [SettingsScreen] for the picker.
+class LocaleStore {
+  LocaleStore(this._store);
+
+  final SecretStore _store;
+
+  static const _key = 'tane.locale';
+
+  /// The saved locale, or null when the user follows the device language.
+  /// Unknown tags (e.g. a locale removed in a later build) also read as null.
+  Future<AppLocale?> saved() async {
+    final raw = await _store.read(_key);
+    if (raw == null || raw.isEmpty) return null;
+    for (final locale in AppLocale.values) {
+      if (locale.name == raw) return locale;
+    }
+    return null;
+  }
+
+  /// Persists [locale] and applies it immediately.
+  Future<void> setLocale(AppLocale locale) async {
+    await _store.write(_key, locale.name);
+    await LocaleSettings.setLocale(locale);
+  }
+
+  /// Forgets the explicit choice and follows the device language from now on.
+  Future<void> useDeviceLocale() async {
+    await _store.write(_key, '');
+    await LocaleSettings.useDeviceLocale();
+  }
+}