Add user creation functionality: implement API endpoint for creating new users, including request handling and response management. Update frontend to support user creation with a new form in the UsersPanel, integrating role selection and input validation. Enhance i18n for new user-related strings.

backend/src/TeleWave.Application/Admin/Users/CreateUser/CreateUserCommand.cs

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+using LiteCqrs;
+using TeleWave.Application.Common.Models;
+
+namespace TeleWave.Application.Admin.Users.CreateUser;
+
+/// <summary>Создание пользователя администратором (в обход открытой регистрации), с выбором роли.</summary>
+public sealed record CreateUserCommand(string UserName, string Password, Guid RoleId)
+    : ICommand<Result<Guid>>;

backend/src/TeleWave.Application/Admin/Users/CreateUser/CreateUserCommandHandler.cs

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+using LiteCqrs;
+using TeleWave.Application.Admin.Roles;
+using TeleWave.Application.Common.Interfaces;
+using TeleWave.Application.Common.Models;
+
+namespace TeleWave.Application.Admin.Users.CreateUser;
+
+public sealed class CreateUserCommandHandler(
+    IIdentityService identityService,
+    IRoleService roleService
+) : ICommandHandler<CreateUserCommand, Result<Guid>>
+{
+    public async Task<Result<Guid>> Handle(
+        CreateUserCommand command,
+        CancellationToken cancellationToken
+    )
+    {
+        // Роль проверяем до создания, чтобы не оставить пользователя с ролью по умолчанию при опечатке.
+        var roles = await roleService.ListRolesAsync(cancellationToken);
+        if (roles.All(r => r.Id != command.RoleId))
+            return Result.Failure<Guid>(RoleErrors.NotFound);
+
+        var createResult = await identityService.CreateUserAsync(
+            command.UserName,
+            command.Password,
+            cancellationToken
+        );
+        if (!createResult.IsSuccess)
+            return Result.Failure<Guid>(createResult.Error);
+
+        var userId = createResult.Value;
+
+        // CreateUserAsync назначает роль по умолчанию — выставляем выбранную админом.
+        var roleResult = await roleService.ChangeUserRoleAsync(userId, command.RoleId, cancellationToken);
+        if (!roleResult.IsSuccess)
+            return Result.Failure<Guid>(roleResult.Error);
+
+        return Result.Success(userId);
+    }
+}

backend/src/TeleWave.Application/Admin/Users/CreateUser/CreateUserCommandValidator.cs

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+using FluentValidation;
+
+namespace TeleWave.Application.Admin.Users.CreateUser;
+
+public sealed class CreateUserCommandValidator : AbstractValidator<CreateUserCommand>
+{
+    public CreateUserCommandValidator()
+    {
+        RuleFor(x => x.UserName).NotEmpty().MinimumLength(3).MaximumLength(64);
+        RuleFor(x => x.Password).NotEmpty().MinimumLength(8);
+        RuleFor(x => x.RoleId).NotEmpty();
+    }
+}