Simple payments block: editor layout broken on small screens

Simple payments block editor breaks at specific breakpoint:

Probably a regression introduced in https://github.com/Automattic/wp-calypso/pull/29027 (cc @rodrigoi )

## Steps to reproduce
- Add simple payments block to post editor
- Make sure sidebar is open
- Set window size to ~830px

Reproduce the simple payments block in the post editor with the sidebar open and the window near the 830px breakpoint. Compare the layout at nearby widths and trace the editor styling associated with the suspected regression from pull request #29027; done means the block remains usable at the reported size without breaking the editor layout.

Written by the indexing model from the issue text.