Set the app bar's height to 42px, following Figma (currently 56px) #2106
Description
Wherever we show an app bar, its height is 56px, just because that's the default with the Material widgets we're working with (see kToolbarHeight).
In all the places the Figma shows an app bar (I think), its height is 42px. For example: https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=6036-294093&m=dev
From a quick experiment, the app bar on a topic-narrow message list isn't ready for the smaller height; the content looks cramped. We can plan to resolve this by doing #1039.
The lightbox app bar also looks cramped with height 42px. I think we can just leave that at 56px—that app bar is already very different from all the other ones, and the lightbox doesn't have a Figma design.