🎨 Palette: Improve accessibility of chat navigation buttons

[thirdeyenation]

💡 What
Added aria-label to the chat navigation icon-only buttons and explicitly set aria-hidden="true" on their inner Material Symbol ligatures in webui/index.html.

🎯 Why
Screen readers would previously read the raw ligature text (like 'vertical_align_top') instead of the intended action, confusing visually impaired users. Providing explicit aria-label attributes ensures screen readers announce the correct action.

📸 Before/After
N/A - This is a purely structural semantic accessibility change with no visual differences.

♿ Accessibility

• Added aria-label attributes to <button> elements for clearer screen reader announcements.
• Hid raw Material Symbol <span class="material-symbols-outlined"> ligatures using aria-hidden="true".

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PR created automatically by Jules for task 7306644765073621219 started by @thirdeyenation

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