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/pantallas/ajustes/pantalla_ajustes_emisoras_personalizadas.dart

@@ -6,6 +6,7 @@ import '../../estado/estado_radio.dart';
 import '../../l10n/display_names.dart';
 import '../../l10n/gen/app_localizations.dart';
 import '../../modelos/emisora.dart';
+import '../../servicios/servicio_anuncios.dart';
 import '../../widgets/pluri_glass_surface.dart';
 import '../../widgets/pluri_layout.dart';
 import '../../widgets/pluri_push_scaffold.dart';
@@ -105,6 +106,11 @@ class _CuerpoEmisorasPersonalizadas extends StatelessWidget {
   }
 
   Future<void> _mostrarFormularioAnadir(BuildContext context) async {
+    // ad-display spec "Interstitial Before Manual Station Add" (design.md
+    // ADR-6): fires on the CTA tap, before the form even opens — a no-op
+    // for premium (ServicioAnuncios' own entitlement gate).
+    await context.read<ServicioAnuncios>().intentarInterstitial();
+    if (!context.mounted) return;
     await showModalBottomSheet(
       context: context,
       isScrollControlled: true,