fix(levels): add fixed height to entire level display pane

Fix slider hover interaction by making the entire level display pane
(including level info, abacus, and digit count) a fixed height. This
prevents the slider from moving when hovering over it.

Changes:
- Add fixed height of 700px on desktop (auto on mobile) to level pane
- Convert container to flexbox with column direction
- Make abacus display area flex: 1 to fill remaining space
- Slider now stays perfectly still under mouse during hover

This makes the hover interaction much more usable - you can now smoothly
move your mouse across the slider without it jumping away.

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

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

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

@@ -253,6 +253,9 @@ export default function LevelsPage() {
                         : 'amber.500',
                 rounded: 'xl',
                 p: { base: '6', md: '8' },
+                height: { base: 'auto', md: '700px' },
+                display: 'flex',
+                flexDirection: 'column',
               })}
             >
               {/* Level Info */}
@@ -300,7 +303,7 @@ export default function LevelsPage() {
                   border: '1px solid',
                   borderColor: 'gray.700',
                   overflowX: 'auto',
-                  minHeight: '400px',
+                  flex: 1,
                 })}
               >
                 <animated.div