🎨 Palette: [UX improvement] Improve accessibility and focus states in Navbar

[belpythons]

💡 What:
Added proper ARIA attributes (aria-expanded, aria-haspopup, aria-label) and explicit visible focus rings (focus-visible:ring-2) to interactive, icon-only buttons in the Navbar.jsx component. This includes the Forum dropdown button, light/dark mode theme toggles, and the mobile hamburger menu button.

🎯 Why:
Previously, these buttons lacked explicit focus states, making it very difficult for keyboard users to know which element was currently focused. Additionally, the lack of aria-expanded and aria-label attributes meant that screen reader users were not informed of the current state of dropdowns or the purpose of icon-only buttons (like the mobile menu).

📸 Before/After:
(Visual changes add a noticeable orange focus ring around the mobile menu toggle when focused via keyboard navigation, as verified locally.)

♿ Accessibility:

• Added aria-haspopup="true" and aria-expanded={isDropdownOpen} to the Forum dropdown.
• Added aria-label (toggling between "Open menu" and "Close menu") and aria-expanded={isOpen} to the mobile menu button.
• Added focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-accent-500 to ensure a consistent, highly visible focus indicator for keyboard users across all icon-only interactive elements in the navbar.

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PR created automatically by Jules for task 13060513485504835409 started by @belpythons

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