Implement username change functionality and enhance Telegram bot registration flow

- Added a new endpoint for changing usernames, allowing users to update their login credentials via the API.
- Integrated username change functionality into the settings page, providing a user-friendly interface for this action.
- Enhanced the Telegram bot to support user registration directly through the bot, including username generation and password delivery.
- Updated documentation to reflect the new username change endpoint and registration flow through the Telegram bot.

docs/api-design.md

@@ -19,6 +19,7 @@ REST поверх HTTP/JSON, авторизация — `Authorization: Bearer <
 | POST  | `/api/auth/refresh`         | —      | — (refresh из cookie)                      | то же, что login; ротация cookie      |
 | POST  | `/api/auth/logout`          | user   | —                                          | `204 No Content`                       |
 | POST  | `/api/auth/change-password` | user   | `{ currentPassword, newPassword }`         | `204 No Content`                       |
+| POST  | `/api/auth/change-username` | user   | `{ newUserName }`                          | `204 No Content`                       |
 | GET   | `/api/auth/me`              | user   | —                                          | `{ id, userName, role, isActivated, telegramLinked }` |
 | DELETE| `/api/auth/me`              | user   | —                                          | `204 No Content`                       |