feat: code viewer scrolling, mobile overview, better footer, mobile scroll up

[danielroe]

use sticky header with page-level scroll instead of nested scroll containers

[Flo0806]

This should be sticky, right? @danielroe

[danielroe]

yes, exactly 👌

[Flo0806]

@danielroe If you prefer this: #59 top button, let me know and I'll remove mine from this PR – then you'll have less stress.