Fix scrolling to older messages with screen reader

[gnprice]

This was found in https://github.com/zulip/zulip-flutter/issues/535#issuecomment-2973276995 when investigating the app's behavior with a screen reader:

Navigating backwards (to older messages) does not work smoothly. Sometimes the focus can get moved to the sticky header instead of the previous message; sometimes the focus moves off-screen or behind the sticky header, instead of automatically scrolled. For reference on the correct behavior, see what happens when you are about to navigate to a channel item on the channels page off-screen.

We should fix this.

A required part of any PR to address this issue will be to try it out with a screen reader and confirm how it behaves. See the description of #535 for instructions.