feat: fade out hover avatar when player stops hovering

Avatar now fades out smoothly when:
- Player moves mouse off all cards
- Player's turn ends
- Player hasn't hovered any card yet

Opacity requires all three conditions: position exists, it's player's turn,
AND player is actively hovering a card (cardElement !== null).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

apps/web/src/app/arcade/matching/components/MemoryGrid.tsx

@@ -116,7 +116,7 @@ function HoverAvatar({
   const springProps = useSpring({
     x: position?.x ?? 0,
     y: position?.y ?? 0,
-    opacity: position && isPlayersTurn ? 1 : 0,
+    opacity: position && isPlayersTurn && cardElement ? 1 : 0,
     config: {
       tension: 280,
       friction: 60,