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/navegacion_auto_test.dart

@@ -240,7 +240,10 @@ void main() {
   group('ConstructorArbolAuto.raiz', () {
     test('con incluirMusicaLocal: true devuelve exactamente 4 carpetas no '
         'reproducibles con los ids esperados', () {
-      final raiz = ConstructorArbolAuto().raiz(incluirMusicaLocal: true);
+      final raiz = ConstructorArbolAuto().raiz(
+        incluirMusicaLocal: true,
+        premium: true,
+      );
 
       expect(raiz, hasLength(4));
       final ids = raiz.map((item) => item.id).toSet();
@@ -262,7 +265,10 @@ void main() {
 
     test('con incluirMusicaLocal: false devuelve exactamente 3 carpetas — '
         'Música Local queda OCULTA, no vacía', () {
-      final raiz = ConstructorArbolAuto().raiz(incluirMusicaLocal: false);
+      final raiz = ConstructorArbolAuto().raiz(
+        incluirMusicaLocal: false,
+        premium: true,
+      );
 
       expect(raiz, hasLength(3));
       final ids = raiz.map((item) => item.id).toSet();
@@ -286,7 +292,7 @@ void main() {
         'segunda vuelta de la misma decisión con evidencia real de uso', () {
       final ids =
           ConstructorArbolAuto()
-              .raiz(incluirMusicaLocal: true)
+              .raiz(incluirMusicaLocal: true, premium: true)
               .map((item) => item.id)
               .toList();