feat: enhance user presence and channel interaction features

- Fix userlist not refreshing after creating a new channel.
- Implement unmute timer with a background job to automatically unmute users.
- Improve userlist display by filtering invisible users and ensuring proper transitions between statuses.
- Add clickable usernames, @mentions, and #channels for easier navigation.
- Embed theme colors from source sites for a more cohesive UI.
- Introduce a stateful userlist that updates incrementally via SignalR events.
- Restrict auto-opening of files to safe types only, enhancing security.
- Refactor user management into a dedicated service to reduce code duplication.
- Add a MuteExpirationService to handle timed mutes.
- Update documentation with Mermaid diagrams for major flows.

src/EchoHub.Tests/Irc/IrcBroadcasterTests.cs

@@ -132,7 +132,7 @@ public class IrcBroadcasterTests
     {
         var (_, bobStream) = AddConnectionWithCapture("bob", "general");
 
-        await _broadcaster.SendUserJoinedAsync("general", "alice");
+        await _broadcaster.SendUserJoinedAsync("general", "alice", null);
 
         var output = bobStream.GetOutputLines();
         Assert.Contains(output, l => l.Contains("JOIN #general") && l.Contains("alice"));
@@ -144,7 +144,7 @@ public class IrcBroadcasterTests
         var (conn, excludedStream) = AddConnectionWithCapture("alice", "general");
         var (_, bobStream) = AddConnectionWithCapture("bob", "general");
 
-        await _broadcaster.SendUserJoinedAsync("general", "alice", conn.ConnectionId);
+        await _broadcaster.SendUserJoinedAsync("general", "alice", null, conn.ConnectionId);
 
         // Excluded connection should not get the message
         Assert.Empty(excludedStream.GetOutputLines());