🎨 Palette: Add accessibility attributes to detail panel close button

[kourdroid]

💡 What: Added an aria-label and title attribute to the "✕" close button in the unit detail panel.
🎯 Why: Icon-only buttons lacking accessible names are invisible to screen readers, and the added title provides a helpful tooltip for mouse users navigating the dense UI.
📸 Before/After: Visual changes are limited to a native browser tooltip appearing on hover over the close button.
♿ Accessibility: Ensures the button is properly announced as "Close detail panel" by screen readers and resolves a basic WCAG issue regarding icon-only controls without labels.

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PR created automatically by Jules for task 15031816121531848244 started by @kourdroid

[google-labs-jules]

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