🎨 Palette: Add accessible labels to detail panel close button

[kourdroid]

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

🎯 Why: Icon-only buttons using text symbols like "✕" lack semantic meaning for screen readers, making it difficult for visually impaired users to understand their function. Additionally, sighted mouse users benefit from tooltips on interactive elements.

📸 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: The aria-label attribute provides an accessible name for screen readers, resolving a potential WCAG issue for icon-only interactive controls. The title attribute adds a standard browser tooltip for hover states.

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PR created automatically by Jules for task 10894196717704423682 started by @kourdroid

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