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.Core/Contracts/IChatBroadcaster.cs

@@ -5,7 +5,7 @@ namespace EchoHub.Core.Contracts;
 public interface IChatBroadcaster
 {
     Task SendMessageToChannelAsync(string channelName, MessageDto message);
-    Task SendUserJoinedAsync(string channelName, string username, string? excludeConnectionId = null);
+    Task SendUserJoinedAsync(string channelName, string username, UserPresenceDto? presence, string? excludeConnectionId = null);
     Task SendUserLeftAsync(string channelName, string username);
     Task SendChannelUpdatedAsync(ChannelDto channel, string? channelName = null);
     Task SendUserStatusChangedAsync(List<string> channelNames, UserPresenceDto presence);

src/EchoHub.Core/Contracts/IEchoHubClient.cs

@@ -8,7 +8,7 @@ namespace EchoHub.Core.Contracts;
 public interface IEchoHubClient
 {
     Task ReceiveMessage(MessageDto message);
-    Task UserJoined(string channelName, string username);
+    Task UserJoined(string channelName, string username, UserPresenceDto? presence);
     Task UserLeft(string channelName, string username);
     Task ChannelUpdated(ChannelDto channel);
     Task UserStatusChanged(UserPresenceDto presence);