feat: custom broadcast buttons UI and fix stale mediaType bug

- Add inline custom button editor (no popups): label, type
  (callback/URL), action value with byte-level validation
- Frontend validates callback_data in UTF-8 bytes via TextEncoder
- URLs restricted to https:// and tg:// matching backend
- Form wrapped in <form> for Enter key submit support
- Fix pre-existing stale mediaType bug in file upload handler
- Translations added for ru, en, zh, fa locales

src/locales/en.json

@@ -1701,7 +1701,14 @@
       "enableEmail": "Email",
       "sendingBoth": "2 broadcasts will be created",
       "bothCreated": "Broadcasts created",
-      "atLeastOneChannel": "Select at least one channel"
+      "atLeastOneChannel": "Select at least one channel",
+      "customButtons": "Custom buttons",
+      "addCustomButton": "Add button",
+      "customButtonLabelPlaceholder": "Button text",
+      "customButtonTypeCallback": "Callback",
+      "customButtonTypeUrl": "URL",
+      "customButtonCallbackPlaceholder": "callback_data (e.g. back_to_menu)",
+      "customButtonUrlPlaceholder": "https://example.com"
     },
     "pinnedMessages": {
       "title": "Pinned Messages",

src/locales/fa.json

@@ -1336,7 +1336,14 @@
       "enableEmail": "Email",
       "sendingBoth": "۲ پیام‌رسانی ایجاد خواهد شد",
       "bothCreated": "پیام‌رسانی‌ها ایجاد شدند",
-      "atLeastOneChannel": "حداقل یک کانال را انتخاب کنید"
+      "atLeastOneChannel": "حداقل یک کانال را انتخاب کنید",
+      "customButtons": "دکمه‌های سفارشی",
+      "addCustomButton": "افزودن دکمه",
+      "customButtonLabelPlaceholder": "متن دکمه",
+      "customButtonTypeCallback": "Callback",
+      "customButtonTypeUrl": "لینک",
+      "customButtonCallbackPlaceholder": "callback_data (مثلاً back_to_menu)",
+      "customButtonUrlPlaceholder": "https://example.com"
     },
     "pinnedMessages": {
       "title": "Pinned Messages",

src/locales/ru.json

@@ -1726,7 +1726,14 @@
       "enableEmail": "Email",
       "sendingBoth": "Будет создано 2 рассылки",
       "bothCreated": "Рассылки созданы",
-      "atLeastOneChannel": "Выберите хотя бы один канал"
+      "atLeastOneChannel": "Выберите хотя бы один канал",
+      "customButtons": "Кастомные кнопки",
+      "addCustomButton": "Добавить кнопку",
+      "customButtonLabelPlaceholder": "Текст кнопки",
+      "customButtonTypeCallback": "Callback",
+      "customButtonTypeUrl": "Ссылка",
+      "customButtonCallbackPlaceholder": "callback_data (например, back_to_menu)",
+      "customButtonUrlPlaceholder": "https://example.com"
     },
     "pinnedMessages": {
       "title": "Закреплённые сообщения",

src/locales/zh.json

@@ -1410,7 +1410,14 @@
       "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"
+      "atLeastOneChannel": "请至少选择一个渠道",
+      "customButtons": "自定义按钮",
+      "addCustomButton": "添加按钮",
+      "customButtonLabelPlaceholder": "按钮文字",
+      "customButtonTypeCallback": "回调",
+      "customButtonTypeUrl": "链接",
+      "customButtonCallbackPlaceholder": "callback_data（例如 back_to_menu）",
+      "customButtonUrlPlaceholder": "https://example.com"
     },
     "pinnedMessages": {
       "title": "置顶消息",