🎨 Palette: Add ARIA labels and aria-hidden to chat nav buttons

[thirdeyenation]

💡 What: Added aria-label attributes to chat navigation buttons (#chat-nav-buttons) based on their title attributes. Also added aria-hidden="true" to their internal <span class="material-symbols-outlined"> elements.

🎯 Why: To improve screen reader accessibility. By adding explicit ARIA labels and hiding the icon spans, screen readers will now cleanly announce "Scroll to top" instead of literally reading out the material icon ligature text "vertical align top".

📸 Before/After: N/A - visual changes were not made. Tested using playwright verifying elements have the correct aria-label and aria-hidden attributes assigned.

♿ Accessibility: Enhanced keyboard and screen reader accessibility for four icon-only chat navigation buttons (Top, Previous user message, Next user message, Bottom).

---

PR created automatically by Jules for task 16748692090866045294 started by @thirdeyenation

[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.