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.Server/Services/PresenceTracker.cs

@@ -14,6 +14,8 @@ public class PresenceTracker
     {
         _connections[connectionId] = (userId, username);
 
+        // Lock is required: ConcurrentDictionary only protects its own slots, not the HashSet values inside.
+        // It also makes the TryGetValue → add sequence atomic to prevent race conditions.
         lock (_lock)
         {
             if (!_userConnections.TryGetValue(username, out var connections))