🎨 Palette: [UX improvement] Add ARIA label to detail panel close button

[kourdroid]

💡 What: Added aria-label and title attributes to the icon-only (✕) close button on the detail panel.

🎯 Why: To improve accessibility for screen reader users and add a helpful tooltip for mouse users, making the action clearer.

📸 Before/After: Visuals remain unchanged, but hover now displays a tooltip and screen readers announce "Close detail panel".

♿ Accessibility: Ensures the icon-only button is properly announced by screen readers.

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PR created automatically by Jules for task 6760188534480605468 started by @kourdroid

[google-labs-jules]

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