🎨 Palette: [Accessibility] Improve screen reader and keyboard UX in Navbar

[belpythons]

🎨 Palette: [Accessibility] Improve screen reader and keyboard UX in Navbar

💡 What:
Added aria-expanded and aria-haspopup attributes to the mobile menu and forum dropdown toggles. Added focus-visible utility classes (focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-accent-500) to the dropdown toggle, mobile menu toggle, and theme toggles.

🎯 Why:
Interactive disclosure widgets (like dropdowns and mobile menus) must communicate their expanded state to assistive technologies, and icon-only buttons require clearly visible focus indicators so that keyboard users can navigate the application.

📸 Before/After:
(Playwright visual verification script verified locally)
Before: Theme toggles and mobile toggles lacked focus rings when navigated via keyboard. Dropdowns didn't communicate state.
After: Active focus-visible rings appear on elements during keyboard navigation, and screen readers read the correct state.

♿ Accessibility:
Improves keyboard navigation visibility (WCAG Focus Visible) and assistive technology state tracking (WCAG Name, Role, Value).

---

PR created automatically by Jules for task 7231954965508750497 started by @belpythons

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.