Update to React 19

[LukasKalbertodt]

There is no pressing need to do that, but we should do it at some point. appkit has been updated to it and further appkit updates require it. We first have to check if our dependencies are all React 19 ready.

Here is a random list of things we could look into improving with React 19. I made it while working on the appkit+studio react 19 update:

• useActionState & useFormStatus
• use for contexts & promises
• ref as normal prop
• Context instead of Context.Provider
• cleanup functions in ref
• useDeferredValue with initialValue
• title, link, meta tags ok everywhere
• better preloading resource support (we should maybe use more of that!)

For reference, here is the Studio update PR: opencast/studio#1269