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_ja.arb

@@ -904,5 +904,13 @@
   "restaurarCompras": "購入を復元",
   "compraError": "購入を完了できませんでした。もう一度お試しください。",
   "restauracionSinCompras": "このアカウントでは以前の購入が見つかりませんでした。",
-  "premiumActivo": "プレミアム有効"
+  "premiumActivo": "プレミアム有効",
+  "premiumHojaTitulo": "PluriWave Premiumのロックを解除",
+  "premiumBeneficioSinAnuncios": "アプリ全体で広告なし",
+  "premiumBeneficioAndroidAuto": "Android Auto",
+  "premiumBeneficioGrabacion": "放送局の録音",
+  "premiumBeneficioVacaciones": "アラームの休暇期間設定",
+  "premiumBeneficioAlarmasIlimitadas": "アラーム数無制限(無料プランは5個まで)",
+  "premiumPagoUnico": "買い切りの一度きりの支払いで永久に使用可能。サブスクリプションではありません。",
+  "premiumAhoraNo": "後で"
 }