fix(worksheets): actually fix dropdown button height by constraining description area

Added minH: '12' to the description div inside the dropdown button.
The previous fix only set minH on the button itself, but the inner
content was still growing/shrinking as scaffolding changed.

Now the description area has a fixed minimum height, preventing
any height changes when moving the difficulty slider.

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

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

apps/web/src/app/create/worksheets/addition/components/ConfigPanel.tsx

@@ -639,6 +639,10 @@ export function ConfigPanel({ formState, onChange }: ConfigPanelProps) {
                                 fontSize: 'xs',
                                 color: isCustom ? 'orange.600' : 'gray.500',
                                 lineHeight: '1.3',
+                                minH: '12',
+                                display: 'flex',
+                                flexDirection: 'column',
+                                gap: '0.5',
                               })}
                             >
                               {isCustom ? (