[BUG] Accessibility: hamburger menu not keyboard friendly

[Seirdy]

Describe the bug

• Using the Tab key to navigate to the hamburger menu is impossible.
• Using the Tab key within the hamburger menu to navigate to the close button is impossible.
• Using the Tab key within the hamburger menu also allows navigating to invisible items obscured by the hamburger menu.

Steps to reproduce the behavior:

1. Set a narrow viewport.
2. Attempt to navigate to the hamburger menu using Tab.
3. Open the menu with the mouse and attempt to navigate to the close button using Tab/Shift+Tab, and observe how hidden elements get selected instead.

A sample site with the hamburger menu enabled is https://simpleprivacy.fr/ (hjttps://github.com/d4rklynk/simpleprivacy.fr)

Additional context

This prevents all Wondermod-based sites that enable the hamburger menu from achieving WCAG A, the most basic level of accessibility conformance set by the WCAG.