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/Telegram/TelegramErrors.cs

@@ -12,4 +12,7 @@ public static class TelegramErrors
 
     public static readonly Error NotLinked =
         Error.Conflict("Telegram.NotLinked", "Telegram не привязан ни к одному аккаунту.");
+
+    public static readonly Error AlreadyLinked =
+        Error.Conflict("Telegram.AlreadyLinked", "Этот Telegram уже привязан к аккаунту — используйте вход через Telegram.");
 }