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/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",