feat: Implement IRC Gateway Service and related functionality

- Add IrcGatewayService to handle IRC connections and commands.
- Create IrcMessage class for parsing IRC protocol lines.
- Implement IrcMessageFormatter for formatting messages as IRC lines.
- Define IrcNumericReply constants for IRC numeric replies.
- Add IrcOptions class for configuration settings related to IRC.
- Create IrcServiceExtensions for adding IRC services to the application.
- Refactor ChannelsController to use IChatService for broadcasting messages and channel updates.
- Update ChatHub to utilize IChatService for user connection and message handling.
- Introduce ChatService to manage chat-related operations and interactions.
- Implement SignalRBroadcaster for broadcasting messages to SignalR clients.
- Update PresenceTracker to retrieve usernames for connections.
- Modify appsettings.example.json to include IRC configuration options.
- Update solution file to include the new IRC project.

src/EchoHub.Core/Contracts/IChatBroadcaster.cs

@@ -0,0 +1,13 @@
+using EchoHub.Core.DTOs;
+
+namespace EchoHub.Core.Contracts;
+
+public interface IChatBroadcaster
+{
+    Task SendMessageToChannelAsync(string channelName, MessageDto message);
+    Task SendUserJoinedAsync(string channelName, string username, string? excludeConnectionId = null);
+    Task SendUserLeftAsync(string channelName, string username);
+    Task SendChannelUpdatedAsync(ChannelDto channel, string? channelName = null);
+    Task SendUserStatusChangedAsync(List<string> channelNames, UserPresenceDto presence);
+    Task SendErrorAsync(string connectionId, string message);
+}

src/EchoHub.Core/Contracts/IChatService.cs

@@ -0,0 +1,36 @@
+using EchoHub.Core.DTOs;
+using EchoHub.Core.Models;
+
+namespace EchoHub.Core.Contracts;
+
+public interface IChatService
+{
+    // Connection lifecycle
+    Task UserConnectedAsync(string connectionId, Guid userId, string username);
+    Task<string?> UserDisconnectedAsync(string connectionId);
+
+    // Channel operations
+    Task<(List<MessageDto> History, string? Error)> JoinChannelAsync(string connectionId, Guid userId, string username, string channelName);
+    Task LeaveChannelAsync(string connectionId, string username, string channelName);
+
+    // Messaging
+    Task<string?> SendMessageAsync(Guid userId, string username, string channelName, string content);
+    Task<List<MessageDto>> GetChannelHistoryAsync(string channelName, int count);
+
+    // Presence
+    Task<string?> UpdateStatusAsync(Guid userId, string username, UserStatus status, string? statusMessage);
+    Task<List<UserPresenceDto>> GetOnlineUsersAsync(string channelName);
+
+    // Broadcasting (used by controllers and IRC gateway)
+    Task BroadcastMessageAsync(string channelName, MessageDto message);
+    Task BroadcastChannelUpdatedAsync(ChannelDto channel, string? channelName = null);
+
+    // Query operations (used by IRC gateway for WHOIS, TOPIC, LIST, AUTH)
+    Task<UserProfileDto?> GetUserProfileAsync(string username);
+    Task<(string? Topic, bool Exists)> GetChannelTopicAsync(string channelName);
+    Task<List<ChannelListItem>> GetChannelListAsync();
+    Task<List<string>> GetChannelsForUserAsync(string username);
+    Task<(Guid UserId, string Username)?> AuthenticateUserAsync(string username, string password);
+}
+
+public record ChannelListItem(string Name, string? Topic, int OnlineCount);