fix(paywall): add dismiss controls and honest premium copy

The premium sheet had no close affordance or way to defer, and its
copy only said "Función Premium" without stating what it unlocks.
Adds a header close (X) button and a "not now" secondary action so
dismissal is never harder than purchasing, and replaces the bare
title with a concrete, honest breakdown of the 5 things premium
unlocks (no ads, Android Auto, station recording, alarm vacation
ranges, unlimited alarms) plus the one-time-purchase framing. The
phone equalizer is never listed, since it stays free for everyone.
New l10n keys added to all 13 locales.

lib/l10n/app_es.arb

@@ -863,5 +863,13 @@
   "restaurarCompras": "Restaurar compras",
   "compraError": "No se ha podido completar la compra. Inténtalo de nuevo.",
   "restauracionSinCompras": "No hemos encontrado ninguna compra anterior en esta cuenta.",
-  "premiumActivo": "Premium activo"
+  "premiumActivo": "Premium activo",
+  "premiumHojaTitulo": "Desbloquea PluriWave Premium",
+  "premiumBeneficioSinAnuncios": "Sin publicidad en toda la app",
+  "premiumBeneficioAndroidAuto": "Android Auto",
+  "premiumBeneficioGrabacion": "Grabación de emisoras",
+  "premiumBeneficioVacaciones": "Rangos de vacaciones para las alarmas",
+  "premiumBeneficioAlarmasIlimitadas": "Alarmas ilimitadas (el plan gratuito permite hasta 5)",
+  "premiumPagoUnico": "Pago único, para siempre. No es una suscripción.",
+  "premiumAhoraNo": "Ahora no"
 }