fix(practice): allow vertical overflow for help overlays

Change overflow: hidden to overflowX: hidden + overflowY: visible
so help elements above the problem are not clipped while still
hiding horizontal transition animation overflow.

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

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

apps/web/src/components/practice/ActiveSession.tsx

@@ -981,7 +981,8 @@ export function ActiveSession({
             display: 'flex',
             justifyContent: 'center',
             width: '100%',
-            overflow: 'hidden', // Clip during transition animation
+            overflowX: 'hidden', // Clip horizontal transition, allow vertical for help overlays
+            overflowY: 'visible',
           })}
         >
           {/* Animated track for problem transitions */}