Split scrolling for navbar and preview window

[gazayas]

Closes #80.

I wanted to use h-full or sc-h-full for these, but it wasn't working as expected. I saw the fixed footer is rendered in https://github.com/bullet-train-co/bullet_train-core/blob/main/bullet_train-themes-light/app/views/showcase/engine/_head.html.erb, I'm curious if that might have part to do with it. Either way, I think it turned out nice.

https://github.com/bullet-train-co/showcase/assets/10546292/8d067cde-90dc-49b4-bd36-453743e3637b