Duplicate session rejection feedback to client

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When a user connects a second time, the router rejects it:
```
if self.users.contains_key(&tenant_user_id) {
let _ = respond_to.send(Err("User already online".to_string()));
return;
}
```

But in session.rs, this error propagates up to ConnectionManager::handle_connection, which just logs it. The WebSocket is already upgraded at that point so the client gets a connection that immediately dies with no explanation. Sending a Message::Close with a reason code/message before dropping the socket would let the client show "You're already connected in another tab."

Start in session.rs at ConnectionManager::handle_connection and trace the duplicate-user error from the router rejection. Check how the upgraded WebSocket is dropped and how Message::Close is represented; done means the client receives a close reason explaining that the user is already connected in another tab.

Written by the indexing model from the issue text.