🎨 Palette: Add ARIA labels to chat navigation buttons

[thirdeyenation]

💡 What: Added aria-label attributes to the chat navigation icon buttons (vertical_align_top, keyboard_arrow_up, keyboard_arrow_down, vertical_align_bottom) in the index.html chat area wrapper.
🎯 Why: These buttons were icon-only and lacked proper labels for screen readers, making navigation inaccessible for assistive technologies. Adding the aria-label ensures the action of the buttons is announced correctly to users relying on screen readers.
📸 Before/After: The visual layout remains identical, but the DOM now includes aria-label="Scroll to top", etc. for the relevant buttons.
♿ Accessibility: Improved screen reader compatibility by explicitly labelling four previously unlabelled icon buttons.

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PR created automatically by Jules for task 6161386746657864674 started by @thirdeyenation

[google-labs-jules]

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