🎨 Palette: [UX improvement] Add ARIA labels to chat navigation buttons

[thirdeyenation]

💡 What: Added aria-label attributes to the chat navigation buttons and aria-hidden="true" to their inner icon spans.
🎯 Why: To improve screen reader accessibility for icon-only action buttons. The raw ligature text of Material Symbols can be confusing if read aloud, and the buttons lacked semantic names.
📸 Before/After: Visual appearance is unchanged.
♿ Accessibility: Screen readers will now announce "Scroll to top", "Previous user message", etc., instead of the icon ligature text.

---

PR created automatically by Jules for task 7820276174609551372 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.