🎨 Palette: Improve Navbar keyboard and screen reader accessibility

[belpythons]

What:
Added aria-expanded and aria-label attributes to the mobile menu toggle and Forum dropdown toggle in Navbar.jsx. Also added consistent focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-accent-500 classes to all interactive theme and menu toggle buttons.

Why:
The mobile menu button was an icon-only button without an aria-label, making it inaccessible to screen reader users. The interactive disclosure widgets (mobile menu and dropdowns) did not communicate their open/closed state (aria-expanded). Furthermore, interactive elements lacked explicit focus indicators, hindering keyboard navigation.

Before/After:

• Before: Mobile menu button had no label. Theme, menu, and dropdown toggles lacked explicit focus rings. Disclosure states were not communicated.
• After: Mobile menu button now has an aria-label. All toggles clearly display a focus ring when navigated via keyboard. Screen readers now announce whether the mobile menu or forum dropdown is expanded or collapsed.

Accessibility:

• Resolved missing ARIA label on an icon-only button.
• Communicated state changes using aria-expanded.
• Enhanced keyboard navigation visibility using focus-visible ring utilities.

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PR created automatically by Jules for task 2168571040023479844 started by @belpythons

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