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.

backend/src/PnvPanel.Application/Common/Interfaces/IIdentityService.cs

@@ -23,6 +23,8 @@ public interface IIdentityService
 
     Task<Result> ChangePasswordAsync(Guid userId, string currentPassword, string newPassword, CancellationToken cancellationToken);
 
+    Task<Result> ChangeUserNameAsync(Guid userId, string newUserName, CancellationToken cancellationToken);
+
     /// <summary>
     /// Помечает пользователя активированным. Изменение не коммитится немедленно (в отличие от
     /// CreateUserAsync/ChangePasswordAsync) — оно попадает в трекер того же DbContext и сохраняется