🎨 Palette: Add ARIA labels to chat navigation buttons

[thirdeyenation]

💡 What: Added aria-label attributes to the icon-only "Scroll to top", "Previous user message", "Next user message", and "Scroll to bottom" buttons in webui/index.html. Added aria-hidden="true" to their internal <span class="material-symbols-outlined"> icon elements.

🎯 Why: To improve accessibility. Screen readers will now announce the purposeful aria-label description of the button (which mirrors the title attribute tooltips) instead of reading out the literal ligature text of the Material icon (e.g. "vertical align top").

📸 Before/After:
(Visuals remain unchanged, changes are purely structural/accessibility-focused in the DOM.)
Before: <button title="Scroll to top"><span class="material-symbols-outlined">vertical_align_top</span></button>
After: <button title="Scroll to top" aria-label="Scroll to top"><span class="material-symbols-outlined" aria-hidden="true">vertical_align_top</span></button>

♿ Accessibility:

• Improved screen reader support for floating chat navigation buttons.
• Prevented assistive technologies from misreading the UI icon ligature text.

---

PR created automatically by Jules for task 14101244070577987078 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.