🎨 Palette: Add accessibility attributes and focus rings to Navbar

[belpythons]

💡 What

This PR enhances the accessibility of the Navbar component. It introduces critical ARIA attributes to the interactive disclosure widgets (the Forum dropdown and the mobile navigation menu) and adds clear, explicit keyboard focus rings to the icon-only buttons (the theme toggles and the mobile menu toggle).

🎯 Why

• Screen readers rely on attributes like aria-expanded and aria-controls to understand and announce the state (open/closed) of dynamic menus. Without them, users navigating via assistive technologies lack context about what their actions are doing.
• Keyboard-only users need strong visual indicators to know which element currently has focus. Icon-only buttons often fail to provide adequate default focus states.

📸 Before/After

A visual verification script was run to confirm that navigating via the Tab key now correctly triggers an accent-500 outline on the previously un-styled icon buttons.

♿ Accessibility

• Added aria-expanded={isDropdownOpen} and aria-haspopup="true" to the Forum dropdown.
• Added aria-expanded={isOpen}, aria-controls="mobile-menu", and explicitly set id="mobile-menu" on the mobile navigation container.
• Added focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-accent-500 to the desktop theme toggle, mobile theme toggle, and mobile menu toggle.

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PR created automatically by Jules for task 16988768704894414862 started by @belpythons

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