messageFix scrolling to older messages with screen reader (#1965)

[Kushagra2024-code]

Fix scrolling to older messages with screen reader

Fixes: #1965
Related: #535

Problem

When using a screen reader on the message list:

Navigating backward to older messages does not behave reliably

Focus may jump to the sticky date header

Focus may move off-screen or behind the sticky header

The view does not always auto-scroll to keep the focused message visible

This breaks expected keyboard/screen-reader navigation.

[MritunjayTiwari14]

This PR makes a large number of extraneous changes that mostly include change from default indentation style that we use in Zulip-flutter. Please see our contributor guidelines, and revise this as best you can to be a highly reviewable PR with clear.

I would suggest you to do a self-review of the code.