Update to React 19 (?)

[davidje13]

Updating is painless, but causes a significant increase in build size:

React 18:
Frontend code size:      273.2kB /   87.8kB gzip /   76.7kB brotli

React 19:
Frontend code size:      313.4kB /   99.7kB gzip /   86.3kB brotli

(40kB extra uncompressed, ~10kB extra compressed)

For now there's no great advantage to updating, so stick to 18. This issue can be used to track places where 19 would be beneficial to see if it becomes worthwhile in the future.

Alternatively it may be worth switching to preact for significant bundle size savings.