🎨 Palette: Add interactive focus and disabled states

[benpiper]

💡 What: Added :focus-visible styles to buttons and inputs, and :disabled styles to buttons. Avoid hover styles on disabled buttons.
🎯 Why: Better keyboard navigation accessibility and better UX for disabled buttons (like when uploading in progress).
📸 Before/After: Visual changes to focus outline for keyboard users, greyed out button for disabled states.
♿ Accessibility: Clearer visual focus indicators for keyboard users.

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PR created automatically by Jules for task 9542455085637287318 started by @benpiper

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