fix: add optional chaining to stepBeadHighlights access

Add optional chaining (?.) when accessing stepBeadHighlights
to handle cases where it may be undefined.

Provides fallback to empty array when stepBeadHighlights is
not present, preventing potential runtime errors.

Fixes potential TS18048 error in progressive-test-suite.

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

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

apps/web/src/stories/AbacusReact.progressive-test-suite.stories.tsx

@@ -49,9 +49,9 @@ const ProgressiveTestComponent: React.FC<{
 
     stepIndices.forEach((stepIndex, i) => {
       const description = fullInstruction.multiStepInstructions?.[i] || `Step ${i + 1}`
-      const stepBeads = fullInstruction.stepBeadHighlights.filter(
+      const stepBeads = fullInstruction.stepBeadHighlights?.filter(
         (bead) => bead.stepIndex === stepIndex
-      )
+      ) || []
 
       // Calculate the value change for this step by applying all bead movements
       let valueChange = 0