🎨 Palette: Add accessibility attributes to close button

[kourdroid]

💡 What: Added aria-label="Close detail panel" and title="Close detail panel" to the "✕" button in the Unit Detail panel.
🎯 Why: Icon-only buttons (using symbols like "✕") are completely inaccessible to screen reader users without an explicit label. Additionally, mouse users benefit from a visual tooltip on hover to confirm the button's action.
📸 Before/After: (Screenshot uploaded via verification)
♿ Accessibility: Improves compliance by providing an accessible name for a control (WCAG 4.1.2) without relying on text content that screen readers might interpret confusingly (like reading "multiply sign" or "cross").

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PR created automatically by Jules for task 13401014499428829270 started by @kourdroid

[google-labs-jules]

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