Handle safe areas (mostly).

[chrisbobbe]

After #4683.

The last commit mentions some shortcomings with the message list; screenshots below. But I think an approach like in that commit is a good enough 80% solution, and getting the message list quite right looks like a bigger project that we don't really have time for right now.

Once the rest of the work looks good, I'd like to add something to our style guide for how we'd like to maintain safe-area handling in the future.

I think the first commit ("i18n: Allow better translations for "# unread message(s)".") could be an independent cleanup, so I can move that to another PR if we want.

Message list after this PR (dark and light mode):

Fixes: #3066