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.

pubspec.yaml

@@ -1,7 +1,7 @@
 name: pluriwave
 description: "Radio mundial con ecualizador, reconocimiento de canciones y UI premium"
 publish_to: 'none'
-version: 1.2.28+150
+version: 1.3.0+151
 
 environment:
   sdk: ^3.7.0
@@ -49,12 +49,13 @@ dependencies:
   geocoding: ^3.0.0
   package_info_plus: ^8.3.1
   
-  # Ads (activar cuando tengamos Ad Unit IDs)
-  # google_mobile_ads: ^5.3.0
-  
+  # Ads — TODO: swap Google test ad unit IDs (servicio_anuncios.dart) for
+  # real AdMob unit IDs once provisioned (iap-freemium-unlock, Open Question).
+  google_mobile_ads: ^5.3.0
+
   # In-app purchase
-  # in_app_purchase: ^3.2.0
-  
+  in_app_purchase: ^3.2.0
+
   # Song recognition (activar con AudD key)
   # permission_handler: ^11.3.1