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/l10n/app_es.arb

@@ -856,5 +856,9 @@
   "alarmDiagnosticsFixAction": "Solucionar",
   "alarmDiagnosticsIntentUnavailable": "No se pudo abrir esa pantalla de ajustes en este teléfono. Buscala manualmente en Ajustes.",
   "alarmDiagnosticsUnavailableHint": "Todavía no pudimos revisar tus ajustes de alarma.",
-  "autoEqDisableOption": "Desactivar"
+  "autoEqDisableOption": "Desactivar",
+  "funcionPremium": "Función Premium",
+  "limiteAlarmasAlcanzado": "Has alcanzado el límite de 5 alarmas gratuitas.",
+  "desbloquearPremium": "Desbloquear Premium",
+  "restaurarCompras": "Restaurar compras"
 }