Show floating date indicator while scrolling in Chats

As an EmbeddedChat developer

I need to display a floating date indicator while scrolling through long conversations

So that users can always understand the date context of the currently visible messages, even during fast scrolling

Context

This functionality is already present in the Rocket.Chat core web client, but is currently missing in EmbeddedChat.

Acceptance Criteria

• A floating date indicator appears at the top of the message list while the user is scrolling
• The indicator updates dynamically when scrolling into messages from a different date
• The indicator disappears shortly after scrolling stops
• The indicator does not block message interaction (non-interactive)
• Existing inline date separators continue to work as before

Demo

Current behavior (indicator not present):

https://github.com/user-attachments/assets/f797d8aa-38cf-4ffa-a159-8ee0579c9261

Expected behavior (indicator present):

https://github.com/user-attachments/assets/379f31db-1824-458d-8b9e-62254852ae81

Compare the EmbeddedChat message-list implementation with the corresponding Rocket.Chat core web client behavior, as identified in the issue. Verify the floating indicator during scrolling, its date updates and dismissal, non-interaction with messages, and preservation of inline date separators.