Connections and Disconnections

User story

As a user, I want the chat application to notify me when someone connects or disconnects from the chat. I expect to see a system message displayed in the chat, merging with other messages based on the sending date and time. Additionally, the online user counter in the header should be updated accordingly.

Acceptance criteria

• When a user connects or disconnects, a system message is displayed in the middle of the chat.
• The system message is merged with other messages based on the sending date and time.
• The online user counter in the header is updated to reflect the current number of online users.