Add floating navigation buttons to super block view

[huyenltnguyen]

Description

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

Demo

Screen.Recording.2025-07-02.at.17.05.31.mov

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