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/en.json

@@ -1099,7 +1099,12 @@
       "emailContent": "Email content",
       "emailContentHint": "HTML supported",
       "emailContentPlaceholder": "<p>Hello, {{user_name}}!</p>\n<p>Your message here...</p>",
-      "emailVariables": "Available variables"
+      "emailVariables": "Available variables",
+      "enableTelegram": "Telegram",
+      "enableEmail": "Email",
+      "sendingBoth": "2 broadcasts will be created",
+      "bothCreated": "Broadcasts created",
+      "atLeastOneChannel": "Select at least one channel"
     },
     "settings": {
       "title": "System Settings",

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",

src/locales/ru.json

@@ -1124,7 +1124,12 @@
       "emailContent": "Содержимое письма",
       "emailContentHint": "Поддерживается HTML",
       "emailContentPlaceholder": "<p>Здравствуйте, {{user_name}}!</p>\n<p>Текст вашего письма...</p>",
-      "emailVariables": "Доступные переменные"
+      "emailVariables": "Доступные переменные",
+      "enableTelegram": "Telegram",
+      "enableEmail": "Email",
+      "sendingBoth": "Будет создано 2 рассылки",
+      "bothCreated": "Рассылки созданы",
+      "atLeastOneChannel": "Выберите хотя бы один канал"
     },
     "settings": {
       "title": "Настройки системы",

src/locales/zh.json

@@ -855,7 +855,12 @@
         "activity": "按活动",
         "source": "按来源",
         "tariff": "按套餐"
-      }
+      },
+      "enableTelegram": "Telegram",
+      "enableEmail": "Email",
+      "sendingBoth": "将创建2个群发",
+      "bothCreated": "群发已创建",
+      "atLeastOneChannel": "请至少选择一个渠道"
     },
     "settings": {
       "title": "系统设置",