🎨 Palette: [Accessibility] Add context to detail panel close button

[kourdroid]

💡 What: Added aria-label and title attributes to the icon-only "✕" close button on the detail panel.
🎯 Why: Icon-only buttons using text symbols like "✕" provide no context for screen readers and no visual tooltip for mouse users hovering over the button.
📸 Before/After: Visuals remain unchanged; DOM element gained aria-label="Close detail panel" and title="Close detail panel".
♿ Accessibility: Improved screen reader support by providing a descriptive accessible name, and enhanced mouse user experience with a native tooltip.

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PR created automatically by Jules for task 6294246737858919669 started by @kourdroid

[google-labs-jules]

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