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/Program.cs

@@ -109,6 +109,7 @@ while (true)
         builder.Services.AddSingleton<LinkEmbedService>();
         builder.Services.AddHostedService<ServerDirectoryService>();
         builder.Services.AddHostedService<FileCleanupService>();
+        builder.Services.AddHostedService<MuteExpirationService>();
 
         // ── Encryption ─────────────────────────────────────────────────────
         builder.Services.AddSingleton<IMessageEncryptionService, MessageEncryptionService>();