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.

test/pantallas/pantalla_ajustes_test.dart

@@ -4,6 +4,7 @@ import 'dart:io';
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:pluriwave/estado/estado_ecualizador.dart';
+import 'package:pluriwave/estado/estado_entitlement.dart';
 import 'package:pluriwave/estado/estado_grabacion.dart';
 import 'package:pluriwave/estado/estado_idioma.dart';
 import 'package:pluriwave/estado/estado_radio.dart';
@@ -50,6 +51,9 @@ void main() {
         ListenableProvider<EstadoEcualizador>.value(value: estado.ecualizador),
         ListenableProvider<EstadoGrabacion>.value(value: estado.grabacion),
         ChangeNotifierProvider<EstadoIdioma>.value(value: estadoIdioma),
+        ChangeNotifierProvider<EstadoEntitlement>(
+          create: (_) => EstadoEntitlement(prefs: null),
+        ),
       ],
       child: MaterialApp(
         locale: const Locale('en'),