🎨 Palette: Add accessibility attributes to detail panel close button

[kourdroid]

💡 What:
Added aria-label="Close detail panel" and title="Close detail panel" to the .detail-close button in index.html.

🎯 Why:
The close button in the detail panel is an icon-only button using a text-symbol ("✕"). Without an ARIA label, screen readers will either announce the symbol phonetically (which can be confusing) or announce "button" with no context. Furthermore, without a title attribute, mouse users do not get a helpful tooltip explaining the button's action.

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

♿ Accessibility:

• Improves screen reader experience by providing an accessible name for an icon-only button.
• Improves visual usability by providing a native tooltip on hover for mouse users.

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PR created automatically by Jules for task 4555019867858298086 started by @kourdroid

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