fix(levels): revert delayed column change, keep overflow hidden

Reverted the delayed column change approach (using displayedIndex state
and onRest callback) which was causing bugs. Went back to the simpler
direct approach where currentIndex immediately drives all changes.

Changes:
- Removed displayedIndex state and displayedLevel variable
- Removed onRest callback from React Spring configuration
- All UI elements now use currentLevel directly
- Kept overflow: 'hidden' on abacus container

The simpler approach provides better UX with immediate feedback while
React Spring still provides smooth scale transitions.

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

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

apps/web/src/app/levels/page.tsx

@@ -357,7 +357,7 @@ export default function LevelsPage() {
                   rounded: 'lg',
                   border: '1px solid',
                   borderColor: 'gray.700',
-                  overflow: 'visible',
+                  overflow: 'hidden',
                   flex: 1,
                 })}
               >