Update to React 19; blockers

[Arnei]

Appkit finally supports React 19, but there are yet more libraries that do not. In particular, react-beforeunload and use-resize-observer still require updates.

react-beforeunload is fortunately small enough that we can just copy the code into our own codebase or write it ourselves if we want to.

use-resize-observer is a bit bigger and requires additional changes beyond bumping the version number. I'm inclined to wait until the project developer finds some spare time, since they are obviously still invested in the project.