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",