feat(rithmomachia): add 80% opacity to guide modal when not hovered

Guide modal now displays at 80% opacity on desktop to allow seeing the game
board behind it. Opacity increases to 100% when hovering over the modal.

Behavior:
- Desktop (≥768px): 80% opacity when not hovered, 100% when hovered
- Mobile (<768px): Always 100% opacity
- Standalone window: Always 100% opacity
- Smooth 0.2s transition between states

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

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

apps/web/src/arcade-games/rithmomachia/components/PlayingGuideModal.tsx

@@ -300,6 +300,9 @@ export function PlayingGuideModal({ isOpen, onClose, standalone = false }: Playi
               height: `${size.height}px`,
               zIndex: Z_INDEX.MODAL,
             }),
+        // 80% opacity on desktop when not hovered, full opacity otherwise
+        opacity: !standalone && window.innerWidth >= 768 && !isHovered ? 0.8 : 1,
+        transition: 'opacity 0.2s ease',
       }}
       onMouseEnter={() => setIsHovered(true)}
       onMouseLeave={() => setIsHovered(false)}