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.
