feat: implement user registration and update SASL authentication handling

src/Directory.Build.props

@@ -1,6 +1,6 @@
 <Project>
   <PropertyGroup>
-    <Version>0.2.7</Version>
+    <Version>0.2.8</Version>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <NoWarn>$(NoWarn);CS1591</NoWarn>
   </PropertyGroup>

src/EchoHub.Core/Contracts/IChatService.cs

@@ -29,4 +29,5 @@ public interface IChatService
     Task<UserProfileDto?> GetUserProfileAsync(string username);
     Task<List<string>> GetChannelsForUserAsync(string username);
     Task<(Guid UserId, string Username)?> AuthenticateUserAsync(string username, string password);
+    Task<(Guid UserId, string Username)?> RegisterUserAsync(string username, string password);
 }

src/EchoHub.Server.Irc/IrcCommandHandler.cs

@@ -170,9 +170,13 @@ public sealed class IrcCommandHandler
 
             var result = await _chatService.AuthenticateUserAsync(username, password);
 
+            // Auth failed — try registering a new account
+            if (result is null)
+                result = await _chatService.RegisterUserAsync(username, password);
+
             if (result is null)
             {
-                _logger.LogWarning("SASL auth failed for user '{Username}' (connection {Id})",
+                _logger.LogWarning("SASL auth/register failed for user '{Username}' (connection {Id})",
                     username, _conn.ConnectionId);
                 await _conn.SendNumericAsync(ServerName, IrcNumericReply.ERR_SASLFAIL,
                     ":SASL authentication failed");
@@ -286,10 +290,14 @@ public sealed class IrcCommandHandler
 
         var result = await _chatService.AuthenticateUserAsync(_conn.Nickname!, _conn.Password);
 
+        // Auth failed — try registering a new account
+        if (result is null)
+            result = await _chatService.RegisterUserAsync(_conn.Nickname!, _conn.Password);
+
         if (result is null)
         {
             await _conn.SendNumericAsync(ServerName, IrcNumericReply.ERR_PASSWDMISMATCH,
-                ":Password incorrect or account not found. Register via the EchoHub client first.");
+                ":Password incorrect.");
             await _conn.SendAsync("ERROR :Authentication failed");
             return;
         }

src/EchoHub.Server/Services/ChatService.cs

@@ -339,6 +339,38 @@ public class ChatService : IChatService
         return (user.Id, user.Username);
     }
 
+    public async Task<(Guid UserId, string Username)?> RegisterUserAsync(string username, string password)
+    {
+        username = username.ToLowerInvariant().Trim();
+
+        if (!ValidationConstants.UsernameRegex().IsMatch(username))
+            return null;
+
+        if (password.Length < 6 || password.Length > ValidationConstants.MaxPasswordLength)
+            return null;
+
+        using var scope = _scopeFactory.CreateScope();
+        var db = scope.ServiceProvider.GetRequiredService<EchoHubDbContext>();
+
+        if (await db.Users.AnyAsync(u => u.Username == username))
+            return null;
+
+        var isFirstUser = !await db.Users.AnyAsync();
+
+        var user = new User
+        {
+            Id = Guid.NewGuid(),
+            Username = username,
+            PasswordHash = BCrypt.Net.BCrypt.HashPassword(password),
+            Role = isFirstUser ? ServerRole.Owner : ServerRole.Member,
+        };
+
+        db.Users.Add(user);
+        await db.SaveChangesAsync();
+
+        return (user.Id, user.Username);
+    }
+
     /// <summary>
     /// Collapse consecutive newlines and cap total line count to prevent newline spam.
     /// </summary>