🎨 Palette: [UX improvement] Add explicit focus rings and ARIA states to interactive nav widgets

[belpythons]

What:
Added explicit ARIA state attributes (aria-expanded, aria-label, aria-haspopup) and visible focus indicators (focus-visible:ring-2) to interactive, icon-only navigation widgets like the mobile menu toggle, theme toggles, and forum dropdowns across Navbar.jsx and FocusLayout.jsx. Also prefixed an unused theme variable to satisfy ESLint.

Why:
Icon-only disclosure widgets and navigation buttons often lack sufficient context and state announcements out-of-the-box, creating significant barriers for screen reader users. Additionally, native browser focus outlines can sometimes be suppressed or insufficiently distinct on custom UI elements, leaving keyboard navigators lost on the page without clear visual feedback.

Before/After:

• Before: Toggling the mobile menu or forum dropdown did not broadcast state changes (aria-expanded) to assistive technologies. Keyboard users navigating via "Tab" received minimal/no visual feedback when landing on icon-only buttons.
• After: Screen readers now correctly announce when these menus open or close. Keyboard users see a distinct, brand-colored focus ring (via focus-visible:ring-accent-500) when tabbing to these buttons, drastically improving the predictability of interaction.

Accessibility:

• Added aria-expanded attributes dynamically bound to state on disclosure buttons.
• Added aria-haspopup to the Forum dropdown button.
• Added explicit, descriptive aria-label attributes where missing.
• Implemented Tailwind focus-visible utility classes to guarantee a high-contrast focus ring for keyboard-only users without polluting pointer/mouse clicks.

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PR created automatically by Jules for task 14391381833891525806 started by @belpythons

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