[🐴] Finalize web message screen

[haileyok]

Stacked on #3866 and #3867

Why

Web requires a little extra logic for maintaining scroll position whenever we load new items at the top of the list. This PR adds a little check whenever the content size changes to see if:

1. We are at the top of the scroll view
2. We have already scrolled at least once (the first scroll takes us to the bottom of the page)

If we meet those criteria, we immediately scroll to the previous offset whenever new items get added to the list.

We also remove an unnecessary wrapper around the List that has become unnecessary after changes in the other two PRs.

Without whitespace

Test Plan

1. Load the messages in a large chat. Verify that you start at the bottom of the conversation.
2. Scroll up to the top. Verify that you see a spinner, and then new messages are added without causing your position to jump.
3. Verify that new messages can be added to the chat while you are scrolled away from the bottom without causing jumps in position.

Screen.Recording.2024-05-05.at.1.52.50.AM.mov

[github-actions]

Old size	New size	Diff
6.87 MB	6.87 MB	1.46 KB (0.02%)

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[estrattonbailey]

Tested on web and Android, looks great

[render]

Your Render PR Server URL is https://social-app-pr-3868.onrender.com.

Follow its progress at https://dashboard.render.com/web/srv-cosfkgicn0vc73fqnu40.