Enhancement: Improve the accessibility of the site header

Currently, there are a few issues with the site's header:

- `` elements are used as buttons but do not contain the required attributes (e.g., `role`, `aria-pressed`) to mark them as such.
- A minimal amount of JavaScript will be required to emulate button functionality.
- `` elements cannot be navigated using the keyboard.
- Adding the attribute `tabindex=0` would fix this and allow all users to navigate to and from it with ease.
- `` elements of first-level navigation items are child elements of toggles (i.e., they are contained within a `` element), rather than sibling elements.
- Such a structure seems a little misguided. Even if it doesn't cause issues, at least for mouse users.
- `` elements are missing interactive visuals (e.g., `cursor: pointer`) that demonstrate their button-like functionality.

Start by locating the site's header markup and styles, then inspect how the label-based toggles and first-level navigation links currently behave. The work is done when the header controls are keyboard-navigable, expose appropriate button semantics, provide interactive visuals, and preserve usable navigation structure.

Written by the indexing model from the issue text.