🎨 Palette: Add ARIA labels and hide ligature text for icon-only buttons

[thirdeyenation]

💡 What: Added aria-label attributes to icon-only buttons in webui/index.html (chat navigation) and webui/components/welcome/welcome-screen.html (welcome banners). Also added aria-hidden="true" to their inner material symbol spans.
🎯 Why: Improves screen reader accessibility. Without aria-label, screen readers have no context for these buttons. Without aria-hidden="true" on the inner icon spans, screen readers will announce the raw ligature text (e.g. reading "vertical align top" instead of the intended accessible name for the button).
📸 Before/After: Visually identical.
♿ Accessibility: Icon-only buttons now have explicitly defined programmatic names for assistive technologies, and the decorative icon ligatures are correctly hidden.

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PR created automatically by Jules for task 3997972035467588213 started by @thirdeyenation

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