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/servicios/servicio_audio_gating_test.dart

@@ -0,0 +1,38 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:pluriwave/servicios/servicio_audio.dart';
+
+/// Android Auto play-path backstop (design.md ADR-4, android-auto-media
+/// spec "Free-Tier Browse Never Leaks Real Content" + "Current-Station
+/// Playback Unaffected By Free Tier"): `playFromMediaId`, `playFromSearch`,
+/// `skipToNext`, `skipToPrevious` must ALL no-op for a free-tier user,
+/// regardless of the target id — gating `getChildren` alone is
+/// insufficient because a head unit caches browse trees, so a stale
+/// `emisora:<uuid>` tap could otherwise bypass browsing entirely. Pure —
+/// `PluriWaveAudioHandler` cannot be instantiated in a unit test (needs a
+/// real platform `AudioPlayer`), so this is the extracted decision the
+/// handler's dispatch methods delegate to (mirrors `mapearEstadoProceso`
+/// and every other pure helper in this file).
+void main() {
+  test('free tier: bloquea cualquier cambio de emisora/salto', () {
+    expect(debeBloquearCambioDeEmisora(premium: false), isTrue);
+  });
+
+  test('premium: nunca bloquea', () {
+    expect(debeBloquearCambioDeEmisora(premium: true), isFalse);
+  });
+
+  group('notificarDesbloqueoAuto / registrarNotificacionDesbloqueoAuto', () {
+    test('sin hook registrado, es un no-op seguro', () {
+      expect(() => notificarDesbloqueoAuto(), returnsNormally);
+    });
+
+    test('invoca el hook registrado exactamente una vez por llamada', () {
+      var llamadas = 0;
+      registrarNotificacionDesbloqueoAuto(() => llamadas++);
+
+      notificarDesbloqueoAuto();
+
+      expect(llamadas, 1);
+    });
+  });
+}