🎨 Palette: Add aria-label to icon-only buttons

[thirdeyenation]

💡 What: Added aria-label attribute to icon-only buttons in the sidebar, chat navigation, and file browser.
🎯 Why: To improve keyboard navigation and screen reader accessibility for users relying on assistive technology.
📸 Before/After: No visual changes, structural HTML changes only.
♿ Accessibility: Made the buttons discoverable with meaningful labels for screen reader users.

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PR created automatically by Jules for task 1683023876369327332 started by @thirdeyenation

[google-labs-jules]

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