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

[kourdroid]

💡 What
Added aria-label="Close detail panel" and title="Close detail panel" to the icon-only (✕) close button within the #detail-panel.

🎯 Why
The button only contained the '✕' symbol, meaning screen readers would read it as "times" or similar, which does not convey its function. The addition of title also creates a helpful tooltip for mouse users who hover over the icon.

📸 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

• Fixes screen reader accessibility for an icon-only button by providing an explicit descriptive aria-label.
• Avoids WCAG 2.5.3 (Label in Name) violation by describing the action rather than the symbol.

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PR created automatically by Jules for task 11970411066758803102 started by @kourdroid

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