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",