🎨 Palette: Add ARIA label and title to detail panel close button

[kourdroid]

💡 What:
Added an aria-label and title attribute to the icon-only "✕" (close) button in the detail panel (.detail-close).

🎯 Why:
Screen readers would previously announce this button poorly or not at all since it only contained a decorative text symbol. Additionally, mouse users lacked context on hover. This change improves both screen reader accessibility and provides a clear visual tooltip.

📸 Before/After:

• Before: <button class="detail-close" onclick="closeDetailPanel()">✕</button>
• After: <button class="detail-close" aria-label="Close detail panel" title="Close detail panel" onclick="closeDetailPanel()">✕</button>

♿ Accessibility:

• Added aria-label="Close detail panel" for screen reader support.
• Added title="Close detail panel" for mouse users.

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PR created automatically by Jules for task 222417503453664263 started by @kourdroid

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