🎨 Palette: [UX improvement] Add ARIA attributes to Navbar menus

[belpythons]

💡 What: Added aria-expanded and aria-controls to the "Forum" dropdown and mobile menu buttons. Added id="mobile-menu" to the mobile menu container.
🎯 Why: Interactive disclosure widgets need to communicate their state to assistive technologies like screen readers to be accessible.
📸 Before/After: Visuals are unchanged. The accessibility tree now correctly reports the expanded state of both menus.
♿ Accessibility: Improves keyboard and screen reader accessibility for the global navigation.

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PR created automatically by Jules for task 14547905396877831679 started by @belpythons

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