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_alarmas.dart

@@ -9,10 +9,12 @@ import '../l10n/app_localizations_ext.dart';
 import '../l10n/gen/app_localizations.dart';
 import '../modelos/alarma_musical.dart';
 import '../modelos/emisora.dart';
+import '../servicios/servicio_anuncios.dart';
 import '../servicios/servicio_programacion_alarmas.dart';
 import '../tema/pluriwave_theme.dart';
 import '../tema/pluriwave_tokens.dart';
 import '../widgets/editor_hora_inline.dart';
+import '../widgets/hoja_premium.dart';
 import '../widgets/pluri_glass_surface.dart';
 import '../widgets/pluri_layout.dart';
 import '../widgets/pluri_push_scaffold.dart';
@@ -105,6 +107,21 @@ class PantallaAlarmas extends StatelessWidget {
     BuildContext context, {
     AlarmaMusical? alarma,
   }) async {
+    // ADR-6 ordering (design.md): for a genuinely NEW alarm (no [alarma]),
+    // the cap-check + maybe-interstitial happen HERE, before the editor
+    // ever opens — "puedeCrearAlarma -> if false, show the limit message
+    // and no ad; if true, maybe-interstitial, then open the editor".
+    // Editing an existing alarm skips both checks entirely: it is never
+    // capped and never triggers the interstitial.
+    if (alarma == null) {
+      final estado = context.read<EstadoAlarmas>();
+      if (!estado.puedeCrearAlarma()) {
+        _mostrarLimiteAlarmas(context);
+        return;
+      }
+      await context.read<ServicioAnuncios>().intentarInterstitial();
+      if (!context.mounted) return;
+    }
     await showModalBottomSheet<void>(
       context: context,
       isScrollControlled: true,
@@ -113,6 +130,22 @@ class PantallaAlarmas extends StatelessWidget {
       builder: (_) => _EditorAlarmaSheet(alarma: alarma),
     );
   }
+
+  /// Freemium-gating spec "Alarm Cap UX Never Bare-Jumps To Paywall": an
+  /// explanatory message with a SECONDARY unlock action — never a direct
+  /// paywall navigation as the sole response to hitting the cap.
+  void _mostrarLimiteAlarmas(BuildContext context) {
+    final l10n = AppLocalizations.of(context);
+    ScaffoldMessenger.of(context).showSnackBar(
+      SnackBar(
+        content: Text(l10n.limiteAlarmasAlcanzado),
+        action: SnackBarAction(
+          label: l10n.desbloquearPremium,
+          onPressed: () => mostrarHojaPremium(context),
+        ),
+      ),
+    );
+  }
 }
 
 class _PanelProximaAlarma extends StatelessWidget {
@@ -1186,8 +1219,34 @@ class _EditorAlarmaSheetState extends State<_EditorAlarmaSheet> {
           sonidoInterno: _sonidoInterno,
           activa: true,
         );
-    await estado.guardarAlarma(alarma);
-    if (mounted) Navigator.pop(context);
+    // The cap-check + interstitial already ran in `PantallaAlarmas
+    // ._abrirEditor` BEFORE this sheet ever opened (ADR-6 ordering: "then
+    // open the editor"). This is only the defense-in-depth backstop against
+    // the state-layer choke point — e.g. a 2nd device created alarms while
+    // this sheet was open — the true authority is `guardarAlarma` itself.
+    final resultado = await estado.guardarAlarma(alarma);
+    if (!mounted) return;
+    if (resultado == ResultadoGuardarAlarma.limiteAlcanzado) {
+      _mostrarLimiteAlarmas(context);
+      return;
+    }
+    Navigator.pop(context);
+  }
+
+  /// Freemium-gating spec "Alarm Cap UX Never Bare-Jumps To Paywall": an
+  /// explanatory message with a SECONDARY unlock action — never a direct
+  /// paywall navigation as the sole response to hitting the cap.
+  void _mostrarLimiteAlarmas(BuildContext context) {
+    final l10n = AppLocalizations.of(context);
+    ScaffoldMessenger.of(context).showSnackBar(
+      SnackBar(
+        content: Text(l10n.limiteAlarmasAlcanzado),
+        action: SnackBarAction(
+          label: l10n.desbloquearPremium,
+          onPressed: () => mostrarHojaPremium(context),
+        ),
+      ),
+    );
   }
 
   List<Emisora> _favoritasConSeleccion(List<Emisora> favoritas) {