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_ajustes.dart

@@ -2,6 +2,7 @@ import 'package:flutter/material.dart';
 import 'package:provider/provider.dart';
 
 import '../estado/estado_ecualizador.dart';
+import '../estado/estado_entitlement.dart';
 import '../estado/estado_grabacion.dart';
 import '../estado/estado_idioma.dart';
 import '../estado/estado_radio.dart';
@@ -10,6 +11,7 @@ import '../l10n/gen/app_localizations.dart';
 import '../modelos/archivo_grabacion.dart';
 import '../modelos/emisora.dart';
 import '../tema/pluriwave_tokens.dart';
+import '../widgets/hoja_premium.dart';
 import '../widgets/pluri_layout.dart';
 import '../widgets/pluri_push_scaffold.dart';
 import '../widgets/pluri_root_header.dart';
@@ -99,6 +101,9 @@ class _AjustesContent extends StatelessWidget {
     final idioma = context.select<EstadoIdioma, Locale?>(
       (e) => e.localeSeleccionado,
     );
+    final esPremium = context.select<EstadoEntitlement, bool>(
+      (e) => e.esPremium,
+    );
 
     return Column(
       children: [
@@ -256,6 +261,18 @@ class _AjustesContent extends StatelessWidget {
         GrupoAjustes(
           titulo: l10n.settingsGroupApplicationTitle,
           filas: [
+            // freemium-gating spec "Settings always shows a premium row":
+            // a persistent buy row (free tier) or a premium-active state
+            // with restore access (premium tier) — both open the same
+            // paywall sheet, which adapts its own body to the tier.
+            FilaAjuste(
+              key: const ValueKey('ajustes-fila-premium'),
+              icon: Icons.workspace_premium_rounded,
+              iconColor: PluriWaveTokens.brand,
+              titulo: l10n.funcionPremium,
+              valor: esPremium ? l10n.equalizerActive : null,
+              onTap: () => mostrarHojaPremium(context),
+            ),
             FilaAjuste(
               icon: Icons.language_rounded,
               titulo: l10n.languageSectionTitle,