feat(iap): add freemium unlock via one-time in-app purchase

Adds a permanent, non-consumable premium unlock (EstadoEntitlement +
PuertoCompras/ServicioComprasPlayBilling) that removes ads and unlocks
alarm vacations, alarms past a 5-alarm free cap, recording start, and
full Android Auto browsing. The phone equalizer stays free for everyone.

- Entitlement is prefs-backed (compra_premium_v1), fail-open, and
  resolvable headlessly via esPremiumPersistido() for the Android Auto
  audio handler, which registers before runApp.
- Android Auto reduced mode keeps the real root folder labels for free
  users; browsing into any of them (and playFromMediaId/playFromSearch/
  skipToNext/skipToPrevious) is blocked at the getChildren/servicio_audio
  choke points, with a locked "Función Premium" item as the backstop.
  Current-station play/pause/stop stays untouched. A free -> premium
  transition actively invalidates the head unit's cached browse tree.
- Ads (top banner + capped interstitial before adding a station or an
  alarm) are gated behind entitlement via ServicioAnuncios, using
  official Google test ad unit IDs pending AdMob provisioning.
- Alarm cap UX shows an explanatory message with a secondary unlock
  action rather than a bare paywall jump; existing data is grandfathered.
- 4 new localization keys translated across all 13 supported locales.

Co-located tests use strict TDD (RED test before implementation) for
every new pure-logic unit; full existing suite passes unchanged.

lib/estado/estado_radio.dart

@@ -47,6 +47,10 @@ class EstadoRadio extends ChangeNotifier {
     Future<File> Function()? resolverArchivoCustom,
     FuenteEmisorasAuto? fuenteAuto,
     bool iniciarAutomaticamente = true,
+    // iap-freemium-unlock (Design ADR-3): threaded straight through to the
+    // internal `EstadoGrabacion` below — `EstadoRadio` itself has no gated
+    // behavior of its own.
+    bool Function()? esPremium,
   }) : audio = audio ?? ServicioAudio(),
        favoritos = favoritos ?? ServicioFavoritos(),
        radio = radio ?? ServicioRadio(),
@@ -66,6 +70,7 @@ class EstadoRadio extends ChangeNotifier {
       servicio: servicioGrabacion ?? ServicioGrabacionRadio(prefs: prefs),
       emisoraActual: () => emisoraActual,
       alError: _errorController.add,
+      esPremium: esPremium,
     );
     busqueda = EstadoBusqueda(
       radio: this.radio,