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/IrcMessage.cs

@@ -0,0 +1,69 @@
+namespace EchoHub.Server.Irc;
+
+/// <summary>
+/// Parsed representation of an IRC protocol line.
+/// Format: [:prefix] COMMAND [params...] [:trailing]
+/// </summary>
+public sealed class IrcMessage
+{
+    public string? Prefix { get; init; }
+    public string Command { get; init; } = "";
+    public List<string> Parameters { get; init; } = [];
+
+    public string? Trailing => Parameters.Count > 0 ? Parameters[^1] : null;
+
+    /// <summary>
+    /// Parse a raw IRC line: [:prefix SPACE] command [SPACE params] CRLF
+    /// </summary>
+    public static IrcMessage Parse(string line)
+    {
+        var span = line.AsSpan().TrimEnd("\r\n");
+        string? prefix = null;
+        var pos = 0;
+
+        // Parse optional prefix
+        if (span.Length > 0 && span[0] == ':')
+        {
+            var spaceIdx = span.IndexOf(' ');
+            if (spaceIdx == -1)
+                return new IrcMessage { Prefix = span[1..].ToString() };
+
+            prefix = span[1..spaceIdx].ToString();
+            pos = spaceIdx + 1;
+        }
+
+        // Skip whitespace
+        while (pos < span.Length && span[pos] == ' ') pos++;
+
+        // Parse command
+        var cmdStart = pos;
+        while (pos < span.Length && span[pos] != ' ') pos++;
+        var command = span[cmdStart..pos].ToString();
+
+        // Parse parameters
+        var parameters = new List<string>();
+        while (pos < span.Length)
+        {
+            while (pos < span.Length && span[pos] == ' ') pos++;
+            if (pos >= span.Length) break;
+
+            if (span[pos] == ':')
+            {
+                // Trailing parameter (rest of line)
+                parameters.Add(span[(pos + 1)..].ToString());
+                break;
+            }
+
+            var paramStart = pos;
+            while (pos < span.Length && span[pos] != ' ') pos++;
+            parameters.Add(span[paramStart..pos].ToString());
+        }
+
+        return new IrcMessage
+        {
+            Prefix = prefix,
+            Command = command,
+            Parameters = parameters,
+        };
+    }
+}