feat: Add superscript to scroll-to-bottom-button.

[tanwar-div]

This feature enhances the scroll-to-bottom button by displaying the number of new, unread messages as a small subscript.
For example, if User A sends 5 new messages while User B is reading earlier messages, the button will show “5” as a subscript.
This improves message visibility and ensures users are aware of unread messages without losing their current reading position.

This feature is inspired from existing WhatsApp feature

Solves #36310

[sentry-io]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: web/src/compose.ts

Function	Unhandled Issue
schedule_message_to_custom_date	Error: Assertion failed / Event Count: 2 Affected Users: 2

[tanwar-div]

Hey @alya
I did pr this feature, would you review it or assign someone to review this (if you like the feature)
Or provide some feedback if you didn't
If you don’t like it, please feel free to close the PR
Thanks

[alya]

Technical review is awaiting further design discussion; please link to the discussion thread here and on the issue you filed.

[tanwar-div]

Discussion thread: #design > New unread feature in scroll-to-bottom button