🎨 Palette: Add ARIA labels to chat navigation buttons

[thirdeyenation]

💡 What: Added aria-label attributes to the chat navigation buttons (scroll to top, previous message, next message, scroll to bottom) and added aria-hidden="true" to their internal Material Symbol span elements.
🎯 Why: To prevent screen readers from confusingly reading the raw ligature text (like "vertical_align_top") instead of the button's actual function.
📸 Before/After: Not a visual change.
♿ Accessibility: Improved screen reader support for critical navigation controls in the chat interface.

---

PR created automatically by Jules for task 8140979936986289282 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.