Add floating navigation buttons to super block view

## Description

The list of blocks is quite long, so I was thinking about adding buttons for navigating to the prev/next block.

Demo

https://github.com/user-attachments/assets/b81790b2-4cc6-48a8-966d-dd43cefa9567

Code: https://dartpad.dev/?id=c61b38e158242f3d6ec2892ca29216b3.

If we go forward with this, we'll need to add the buttons to:
- The super block view of block-based super blocks
- The module view of chapter-based super blocks