🎨 Palette: [UX improvement] Add ARIA attributes and focus rings to Navbar toggle buttons

[belpythons]

💡 What

Added missing ARIA attributes (aria-expanded and aria-label) and explicit keyboard focus rings (focus-visible:ring-2 focus-visible:ring-accent-500) to the interactive toggle buttons in the Navbar component (Forum dropdown, Theme toggles, and mobile menu toggle). Cleaned up a minor unused variable warning.

🎯 Why

Icon-only buttons and interactive disclosure widgets (like dropdowns and mobile menus) frequently fail to communicate their state to screen readers and often lack clear visual indicators when navigated via keyboard. Adding explicit ARIA attributes and focus rings ensures a significantly better experience for users relying on assistive technologies or keyboard navigation.

📸 Before/After

• Before: Tabbing through the navbar left users with no clear visual indication of which element was focused, especially on the icon-only toggle buttons. Screen readers were not informed if the mobile menu or forum dropdown was currently expanded.
• After: Keyboard users now see a clear, high-contrast focus ring around the active button. Screen readers now correctly announce the expanded/collapsed state of the dropdown and mobile menu.

♿ Accessibility

• Added aria-expanded to the Forum dropdown and mobile menu toggle buttons.
• Added an explicit aria-label to the mobile menu button that updates based on its state ("Open menu" / "Close menu").
• Added focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-accent-500 to all toggle buttons in the navbar.

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PR created automatically by Jules for task 18004634736279953681 started by @belpythons

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