feat(worksheets): Phase 10 - Add operator validation

Adds validation for operator field in worksheet config:
- Validate operator is one of: addition, subtraction, or mixed
- Add clarifying comment that digit range applies to both operations
- All other validation rules apply uniformly to both operators

Completes the 10-phase implementation plan for subtraction support.
The worksheet creator now fully supports:
- Addition only
- Subtraction only
- Mixed mode (50/50 addition and subtraction)

All phases complete:
1. ✅ Operator selection UI
2. ✅ Subtraction problem generation
3. ✅ Problem analysis for smart mode
4. ✅ Typst rendering with borrow boxes
5. ✅ Unified typstGenerator dispatch
6. ✅ Display rules for both operators
7. ✅ Auto-save persistence
8. ✅ Preview/example routes
9. ✅ DisplayOptionsPreview component
10. ✅ Validation

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

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

apps/web/src/app/create/worksheets/addition/validation.ts

@@ -56,6 +56,7 @@ export function validateWorksheetConfig(formState: WorksheetFormState): Validati
   }
 
   // V4: Validate digitRange (min and max must be 1-5, min <= max)
+  // Note: Same range applies to both addition and subtraction
   const digitRange = formState.digitRange ?? { min: 2, max: 2 }
   if (!digitRange.min || digitRange.min < 1 || digitRange.min > 5) {
     errors.push('Digit range min must be between 1 and 5')
@@ -67,6 +68,12 @@ export function validateWorksheetConfig(formState: WorksheetFormState): Validati
     errors.push('Digit range min cannot be greater than max')
   }
 
+  // V4: Validate operator (addition, subtraction, or mixed)
+  const operator = formState.operator ?? 'addition'
+  if (!['addition', 'subtraction', 'mixed'].includes(operator)) {
+    errors.push('Operator must be "addition", "subtraction", or "mixed"')
+  }
+
   // Validate seed (must be positive integer)
   const seed = formState.seed ?? Date.now() % 2147483647
   if (!Number.isInteger(seed) || seed < 0) {