Fixed React/Next.js bugs; chapter/step display works.

I'm going to try and finish what I started yesterday.

I'm stuck on some nextjs or react bullshit. The button doesn't work, and there are no errors to check why.

Turns out I was using the same open state for both selectors after refactoring.

I fixed that, as well as some other bugs, and now I can change chapters and steps and they display as expected.

I can see that some parts don't render code properly, but I'll leave that for tomorrow.