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.Client/Services/ConnectionManager.cs

@@ -35,7 +35,7 @@ internal sealed class ConnectionManager : IAsyncDisposable
     // ── Events (forwarded from SignalR) ───────────────────────────────────
 
     public event Action<MessageDto>? MessageReceived;
-    public event Action<string, string>? UserJoined;
+    public event Action<string, string, UserPresenceDto?>? UserJoined;
     public event Action<string, string>? UserLeft;
     public event Action<UserPresenceDto>? UserStatusChanged;
     public event Action<string, string, string?>? UserKicked;
@@ -234,7 +234,7 @@ internal sealed class ConnectionManager : IAsyncDisposable
     private void WireConnectionEvents(EchoHubConnection connection)
     {
         connection.OnMessageReceived += msg => MessageReceived?.Invoke(msg);
-        connection.OnUserJoined += (ch, user) => UserJoined?.Invoke(ch, user);
+        connection.OnUserJoined += (ch, user, presence) => UserJoined?.Invoke(ch, user, presence);
         connection.OnUserLeft += (ch, user) => UserLeft?.Invoke(ch, user);
         connection.OnUserStatusChanged += p => UserStatusChanged?.Invoke(p);
         connection.OnUserKicked += (ch, user, reason) => UserKicked?.Invoke(ch, user, reason);