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/pantalla_vacaciones.dart

@@ -8,6 +8,7 @@ import '../l10n/gen/app_localizations.dart';
 import '../modelos/alarma_musical.dart';
 import '../tema/pluriwave_theme.dart';
 import '../tema/pluriwave_tokens.dart';
+import '../widgets/hoja_premium.dart';
 import '../widgets/pluri_glass_surface.dart';
 import '../widgets/pluri_layout.dart';
 import '../widgets/pluri_push_scaffold.dart';
@@ -927,7 +928,14 @@ class _EditorVacacionesSheetState extends State<_EditorVacacionesSheet> {
         fin: _fin,
         nombre: nombre,
       );
-      await estado.crearRangoVacaciones(rango);
+      // freemium-gating spec "Gated Feature Set": vacation creation is
+      // fully gated (unlike the alarm cap, there is no free allowance) —
+      // `crearRangoVacaciones` is the authoritative choke point.
+      final creada = await estado.crearRangoVacaciones(rango);
+      if (!creada) {
+        if (mounted) await mostrarHojaPremium(context);
+        return;
+      }
     }
     if (mounted) Navigator.pop(context);
   }