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_zh.arb

@@ -904,5 +904,13 @@
   "restaurarCompras": "恢复购买",
   "compraError": "无法完成购买，请重试。",
   "restauracionSinCompras": "未在此账户中找到以前的购买记录。",
-  "premiumActivo": "高级版已解锁"
+  "premiumActivo": "高级版已解锁",
+  "premiumHojaTitulo": "解锁 PluriWave Premium",
+  "premiumBeneficioSinAnuncios": "全应用无广告",
+  "premiumBeneficioAndroidAuto": "Android Auto",
+  "premiumBeneficioGrabacion": "电台录音",
+  "premiumBeneficioVacaciones": "闹钟的假期时间段",
+  "premiumBeneficioAlarmasIlimitadas": "无限闹钟(免费版最多支持5个)",
+  "premiumPagoUnico": "一次性付费,永久使用,不是订阅。",
+  "premiumAhoraNo": "以后再说"
 }