Convert Accounts module to MediatR commands/queries

Replaces IAccountService/AccountService with one command/query per
operation, following the pattern set in the Auth module.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

mws.application/Accounts/Commands/CreateAccount.cs

@@ -0,0 +1,54 @@
+using AutoMapper;
+using MediatR;
+using Mws.Application.Auth;
+using Mws.Application.Common;
+using Mws.Application.Permissions;
+using Mws.Domain.Users;
+
+namespace Mws.Application.Accounts;
+
+public record CreateAccountCommand(Guid ActorUserId, CreateAccountRequest Request) : IRequest<AccountDto>;
+
+public class CreateAccountHandler(IUnitOfWork uow, IPasswordHasher passwordHasher, IPermissionService permissions, IMapper mapper)
+    : IRequestHandler<CreateAccountCommand, AccountDto>
+{
+    private const string Screen = "accounts";
+
+    public async Task<AccountDto> Handle(CreateAccountCommand command, CancellationToken ct)
+    {
+        await permissions.EnsureAsync(command.ActorUserId, Screen, PermissionAction.Create, ct);
+
+        var request = command.Request;
+        var username = request.Username.Trim();
+        if (string.IsNullOrWhiteSpace(username) || string.IsNullOrWhiteSpace(request.Password))
+        {
+            throw new BadRequestException("Username and password are required");
+        }
+
+        if (await uow.Users.ExistsByUsernameAsync(username, ct))
+        {
+            throw new BadRequestException("Username already exists");
+        }
+
+        var role = await uow.Roles.GetByIdAsync(request.RoleId, ct)
+            ?? throw new BadRequestException("Role not found");
+
+        var now = DateTime.UtcNow;
+        var user = new User
+        {
+            Id = Guid.NewGuid(),
+            Username = username,
+            PasswordHash = passwordHasher.Hash(request.Password),
+            DisplayName = request.DisplayName.Trim(),
+            RoleId = role.Id,
+            IsActive = true,
+            CreatedAt = now,
+            UpdatedAt = now,
+        };
+
+        uow.Users.Add(user);
+        await uow.SaveChangesAsync(ct);
+        user.Role = role;
+        return mapper.Map<AccountDto>(user);
+    }
+}