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_en.arb

@@ -904,5 +904,13 @@
   "restaurarCompras": "Restore purchases",
   "compraError": "We couldn't complete the purchase. Please try again.",
   "restauracionSinCompras": "We didn't find any previous purchase on this account.",
-  "premiumActivo": "Premium active"
+  "premiumActivo": "Premium active",
+  "premiumHojaTitulo": "Unlock PluriWave Premium",
+  "premiumBeneficioSinAnuncios": "No ads anywhere in the app",
+  "premiumBeneficioAndroidAuto": "Android Auto",
+  "premiumBeneficioGrabacion": "Station recording",
+  "premiumBeneficioVacaciones": "Vacation ranges for alarms",
+  "premiumBeneficioAlarmasIlimitadas": "Unlimited alarms (the free plan allows up to 5)",
+  "premiumPagoUnico": "One-time purchase, forever. Not a subscription.",
+  "premiumAhoraNo": "Not now"
 }