🎨 Palette: Add ARIA labels to icon-only buttons

[thirdeyenation]

💡 What: Added aria-label to icon-only buttons (like chat nav, task actions, and file actions) and set aria-hidden="true" on the inner Google Material ligature icons.
🎯 Why: Screen readers cannot infer the meaning of an icon button without an ARIA label, and without aria-hidden="true", they often read the literal ligature text (e.g., "close", "keyboard_arrow_down"), causing severe confusion for users navigating via keyboard and screen reader.
📸 Before/After: N/A - Visuals are unchanged.
♿ Accessibility: Massively improves screen reader support by ensuring interactive elements announce their purpose clearly, rather than relying solely on hover-based title attributes which don't work reliably for all ATs.

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PR created automatically by Jules for task 3628953578418800057 started by @thirdeyenation

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