🎨 Palette: [UX improvement] Add ARIA labels and hide ligatures on chat navigation buttons

[thirdeyenation]

💡 What: Added aria-label to the 4 floating chat navigation icon buttons and added aria-hidden="true" to their internal <span> Material Symbol ligatures.
🎯 Why: Icon-only buttons using ligatures can be read out confusingly by screen readers (e.g. reading the raw text "vertical align top"). This correctly identifies the button's purpose without doubling up or relying solely on title attributes.
📸 Before/After: Visuals remain identical, but DOM is now accessible.
♿ Accessibility: Correctly maps Material Icons for screen readers in webui/index.html.

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PR created automatically by Jules for task 7068876869206784631 started by @thirdeyenation

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