feat: dual-channel broadcast form (Telegram + Email simultaneously)

Replace single channel radio with independent toggles allowing both
channels to be enabled at once with separate audiences and content.
When both enabled, sends two sequential API requests.

src/locales/fa.json

@@ -803,7 +803,12 @@
         "activity": "بر اساس فعالیت",
         "source": "بر اساس منبع",
         "tariff": "بر اساس تعرفه"
-      }
+      },
+      "enableTelegram": "Telegram",
+      "enableEmail": "Email",
+      "sendingBoth": "۲ پیام‌رسانی ایجاد خواهد شد",
+      "bothCreated": "پیام‌رسانی‌ها ایجاد شدند",
+      "atLeastOneChannel": "حداقل یک کانال را انتخاب کنید"
     },
     "payments": {
       "title": "تأیید پرداخت",