Mobile: Navigation Menu Hidden by <nav> Element

# Bug Description

When the mobile navigation menu (accessed via the burger icon) is opened on a small viewport, the main element on the page is incorrectly rendered above the menu overlay. This is a z-index issue that causes a significant portion of the mobile menu to be obscured, making it unusable for users.

https://github.com/user-attachments/assets/14911e7d-3512-4c94-a163-075cd84e3f50

Reproduce the burger-menu overlay at a small viewport, then trace the mobile navigation and the main element in the frontend styles. Check the z-index rules affecting both layers. Done means the opened mobile menu remains fully above the page navigation and is usable without obscured content.

Written by the indexing model from the issue text.