Center "Open Chat" button in chat panel

The layout of the chat panel wasn't correct but this visual glitch
was being masked by caching, because it seems like Taffy was rendering
things slightly differently when laying out the chat panel as a detached
layout node. This wasn't an issue with caching, but rather an inconsistency
with how Taffy lays things out.

Co-Authored-By: Thorsten <thorsten@zed.dev>
Co-Authored-By: Piotr <piotr@zed.dev>