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.Server.Irc/IrcBroadcaster.cs

@@ -0,0 +1,67 @@
+using EchoHub.Core.Contracts;
+using EchoHub.Core.DTOs;
+
+namespace EchoHub.Server.Irc;
+
+public class IrcBroadcaster(IrcGatewayService gateway) : IChatBroadcaster
+{
+    public async Task SendMessageToChannelAsync(string channelName, MessageDto message)
+    {
+        var lines = IrcMessageFormatter.FormatMessage(message);
+
+        foreach (var conn in gateway.GetConnectionsInChannel(channelName))
+        {
+            // IRC convention: don't echo sender's own message
+            if (conn.Nickname == message.SenderUsername)
+                continue;
+
+            foreach (var line in lines)
+                await conn.SendAsync(line);
+        }
+    }
+
+    public async Task SendUserJoinedAsync(string channelName, string username, string? excludeConnectionId = null)
+    {
+        foreach (var conn in gateway.GetConnectionsInChannel(channelName))
+        {
+            if (conn.ConnectionId == excludeConnectionId) continue;
+            await conn.SendAsync($":{username}!{username}@echohub JOIN #{channelName}");
+        }
+    }
+
+    public async Task SendUserLeftAsync(string channelName, string username)
+    {
+        foreach (var conn in gateway.GetConnectionsInChannel(channelName))
+        {
+            if (conn.Nickname == username) continue;
+            await conn.SendAsync($":{username}!{username}@echohub PART #{channelName}");
+        }
+    }
+
+    public async Task SendChannelUpdatedAsync(ChannelDto channel, string? channelName = null)
+    {
+        var target = channelName ?? channel.Name;
+        if (channel.Topic is null) return;
+
+        foreach (var conn in gateway.GetConnectionsInChannel(target))
+        {
+            await conn.SendAsync($":{gateway.Options.ServerName} TOPIC #{channel.Name} :{channel.Topic}");
+        }
+    }
+
+    public Task SendUserStatusChangedAsync(List<string> channelNames, UserPresenceDto presence)
+    {
+        // IRC has no active status broadcast. Clients discover away via WHOIS/WHO.
+        return Task.CompletedTask;
+    }
+
+    public async Task SendErrorAsync(string connectionId, string message)
+    {
+        if (!connectionId.StartsWith("irc-")) return;
+
+        if (gateway.Connections.TryGetValue(connectionId, out var conn))
+        {
+            await conn.SendAsync($":{gateway.Options.ServerName} NOTICE {conn.Nickname ?? "*"} :{message}");
+        }
+    }
+}