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/Auth/ChangeUserName/ChangeUserNameCommand.cs

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+using PnvPanel.Application.Common.Messaging;
+using PnvPanel.Application.Common.Models;
+
+namespace PnvPanel.Application.Auth.ChangeUserName;
+
+public sealed record ChangeUserNameCommand(string NewUserName) : ICommand<Result>;

backend/src/PnvPanel.Application/Auth/ChangeUserName/ChangeUserNameCommandHandler.cs

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+using PnvPanel.Application.Common.Interfaces;
+using PnvPanel.Application.Common.Messaging;
+using PnvPanel.Application.Common.Models;
+
+namespace PnvPanel.Application.Auth.ChangeUserName;
+
+public sealed class ChangeUserNameCommandHandler(IIdentityService identityService, ICurrentUser currentUser)
+    : ICommandHandler<ChangeUserNameCommand, Result>
+{
+    public Task<Result> Handle(ChangeUserNameCommand command, CancellationToken cancellationToken)
+    {
+        if (currentUser.UserId is not { } userId)
+            return Task.FromResult(Result.Failure(AuthErrors.Unauthorized));
+
+        return identityService.ChangeUserNameAsync(userId, command.NewUserName, cancellationToken);
+    }
+}

backend/src/PnvPanel.Application/Auth/ChangeUserName/ChangeUserNameCommandValidator.cs

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+using FluentValidation;
+
+namespace PnvPanel.Application.Auth.ChangeUserName;
+
+public sealed class ChangeUserNameCommandValidator : AbstractValidator<ChangeUserNameCommand>
+{
+    public ChangeUserNameCommandValidator()
+    {
+        RuleFor(x => x.NewUserName)
+            .NotEmpty()
+            .Length(3, 32)
+            .Matches("^[a-zA-Z0-9_.-]+$")
+            .WithMessage("Имя пользователя может содержать только латиницу, цифры, '_', '.', '-'.");
+    }
+}