🎨 Palette: [UX improvement] Improve Navbar Accessibility

[belpythons]

What: Added aria-expanded and aria-haspopup attributes, as well as focus-visible outline styles, to the mobile menu and dropdown toggles in the Navbar. Also fixed an unused variable linting error.

Why: To ensure that interactive disclosure widgets are fully accessible to screen reader users and can be easily navigated using a keyboard.

Before/After:

• Before: The mobile menu toggle, dropdown button, and theme toggles lacked explicit ARIA attributes indicating their expanded state or popup availability, and lacked visible focus rings.
• After: These elements now broadcast their states properly and show a distinct ring when focused via keyboard navigation.

Accessibility: This directly addresses WCAG standards for Name, Role, Value (4.1.2) by providing state information (aria-expanded) to assistive technologies and Focus Visible (2.4.7) by ensuring keyboard users can see which element has focus.

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PR created automatically by Jules for task 10066964687456004852 started by @belpythons

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