fix(worksheets): Add operator to preview query key and update UI labels

Two fixes:

1. **Preview cache invalidation**: Add formState.operator to WorksheetPreview
   query key so preview refreshes when switching between addition/subtraction/mixed

2. **Dynamic UI labels**: Update "Carry Boxes" label to show:
   - "Carry Boxes" for addition mode
   - "Borrow Boxes" for subtraction mode
   - "Carry/Borrow Boxes" for mixed mode

   Description text also updates to match operator type.

Scaffolding for borrowing:
- Borrow boxes (diagonal split boxes showing source → destination)
- Displayed FROM higher place TO lower place (opposite of carries)
- Same underlying field (showCarryBoxes) controls both

🤖 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

@@ -2241,8 +2241,20 @@ export function ConfigPanel({ formState, onChange }: ConfigPanelProps) {
                   <ToggleOption
                     checked={formState.showCarryBoxes ?? true}
                     onChange={(checked) => onChange({ showCarryBoxes: checked })}
-                    label="Carry Boxes"
-                    description="Help students track regrouping during addition"
+                    label={
+                      formState.operator === 'subtraction'
+                        ? 'Borrow Boxes'
+                        : formState.operator === 'mixed'
+                          ? 'Carry/Borrow Boxes'
+                          : 'Carry Boxes'
+                    }
+                    description={
+                      formState.operator === 'subtraction'
+                        ? 'Help students track borrowing during subtraction'
+                        : formState.operator === 'mixed'
+                          ? 'Help students track regrouping (carrying in addition, borrowing in subtraction)'
+                          : 'Help students track regrouping during addition'
+                    }
                   />
 
                   <ToggleOption

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

@@ -92,6 +92,8 @@ function PreviewContent({ formState, initialData }: WorksheetPreviewProps) {
       // V4: Problem size (CRITICAL - affects column layout and problem generation)
       formState.digitRange?.min,
       formState.digitRange?.max,
+      // V4: Operator selection (addition, subtraction, or mixed)
+      formState.operator,
       // V4: Mode and conditional display settings
       formState.mode,
       formState.displayRules, // Smart mode: conditional scaffolding