feat: display a floating date indicator in the chat body when scrolling.

[Khizarshah01]

Show floating date indicator while scrolling

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Acceptance Criteria fulfillment

• A floating date indicator appears at the top of the message list while 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
• Existing inline date separators continue to function as before

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Fixes #1115

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Video/Screenshots

Before (indicator not present):

Screencast.from.2026-01-27.00-15-28.mp4

After (indicator present):

Screencast.from.2026-01-27.00-14-18.mp4

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PR Test Details

• Tested in EmbeddedChat with long conversations
• Confirmed correct date updates when crossing date separators
• Confirmed no interaction issues with messages while indicator is visible

Note: The PR will be ready for live testing at
https://rocketchat.github.io/EmbeddedChat/pulls/pr-<pr_number>
after approval. Contributors are requested to replace <pr_number> with the actual PR number.

[Khizarshah01]

@Spiral-Memory please review when convenient
thank you!