[FEAT] Update the way the nav height is calculated in the sidebar viewport component

[xelaint]

💡 Feature request

Feature Name

The Desired Behavior

--daff-sidebar-side-fixed-top-shift is currently a fixed value, but this causes issues when a border bottom is added to the viewport nav. The sidebar ends up hiding the bottom border.

Your Use Case

As a designer, I'd like to be able to add a border-bottom to the daffio-header and have the --daff-sidebar-side-fixed-top-shift value adjust accordingly.

Prior Work

Environment

Daffodil version: X.Y.Z
Angular version: X.Y.Z 

 
For Tooling issues:
- Node version: XX  
- Platform:  

Others: