Add unread messages divider

[srijnabhargav]

As an EmbeddedChat developer

I need to:
add an "Unread Messages" divider that appears above the first unread message when new messages arrive while the user is scrolled up.

So That:
users can clearly identify where unread messages begin, consistent with Rocket.Chat behavior.

Acceptance Criteria

• Show an "Unread Messages" divider above the first unread message after clicking the "New Messages" popup
• Ensure only one divider is rendered even when multiple unread messages exist
• Remove the divider on user activity (send message, reopen chat, scroll to bottom)

RC reference