🎨 Palette: Enhance Navbar interactive element accessibility

[belpythons]

💡 What: Added aria-expanded and aria-label attributes to the Navbar dropdown and mobile menu buttons. Added explicit focus-visible styling (focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-accent-500) to all icon-only interactive elements (theme toggle and menu toggle).
🎯 Why: Improves screen reader accessibility by effectively communicating the expanded/collapsed state of menus, and vastly improves keyboard navigation accessibility by providing a clear, high-contrast focus ring on interactive icon buttons that previously lacked visual feedback when focused.
📸 Before/After: Visual regression testing verified via Playwright that layout remains unchanged, while keyboard focus clearly highlights the buttons.
♿ Accessibility: Fixes missing expanded state communication for dropdown menus and enhances keyboard focus visibility in accordance with WCAG guidelines.

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PR created automatically by Jules for task 8130865357213102417 started by @belpythons

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