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.Infrastructure/Identity/IdentityService.cs

@@ -77,6 +77,23 @@ internal sealed class IdentityService(UserManager<AppUser> userManager, SignInMa
                 string.Join("; ", result.Errors.Select(e => e.Description))));
     }
 
+    public async Task<Result> ChangeUserNameAsync(Guid userId, string newUserName, CancellationToken cancellationToken)
+    {
+        var user = await userManager.FindByIdAsync(userId.ToString());
+        if (user is null)
+            return Result.Failure(AuthErrors.Unauthorized);
+
+        var result = await userManager.SetUserNameAsync(user, newUserName);
+        if (result.Succeeded)
+            return Result.Success();
+
+        return result.Errors.Any(e => e.Code == nameof(IdentityErrorDescriber.DuplicateUserName))
+            ? Result.Failure(AuthErrors.DuplicateUserName)
+            : Result.Failure(Error.Validation(
+                "Auth.UserNameChangeFailed",
+                string.Join("; ", result.Errors.Select(e => e.Description))));
+    }
+
     public async Task<Result> ActivateUserAsync(Guid userId, Guid activatedBy, CancellationToken cancellationToken)
     {
         var user = await userManager.FindByIdAsync(userId.ToString());