fix: zarinpal silent failure + show payment error in checkout

Previously the subscribe mutation had no onError handler, so any
payment initiation failure (wrong merchant ID, ZarinPal API error,
disabled payment method) would silently re-enable the button with
no user feedback. Now errors are shown below the Pay button.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/dashboard/messages/ar.json

@@ -1019,7 +1019,8 @@
       "total": "المبلغ المستحق",
       "secureNote": "تتم المعالجة عبر بوابة دفع بنكية آمنة.",
       "payTotal": "ادفع {total}",
-      "redirecting": "جارٍ التحويل إلى البوابة..."
+      "redirecting": "جارٍ التحويل إلى البوابة...",
+      "paymentFailed": "فشل الدفع. الرجاء المحاولة مرة أخرى."
     }
   },
   "settings": {