fix: Custom step navigation bugs - go-back, descendants, redundant checkbox

- Show previously-created custom steps as clickable options on decision
  nodes so they remain accessible after going back
- Fix breadcrumb to show custom step titles instead of raw UUIDs
- Fix ContinuationModal to show grandchildren (two levels deep) instead
  of immediate children that duplicate option labels
- Remove redundant "Save to Library" checkbox from StepForm since
  PostStepActionModal now handles that decision

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/components/step-library/CustomStepModal.tsx

@@ -36,9 +36,7 @@ export function CustomStepModal({ isOpen, onClose, onInsertStep }: CustomStepMod
 
   if (!isOpen) return null
 
-  const handleFormSubmit = async (data: StepCreate, _saveToLibrary: boolean) => {
-    // Note: saveToLibrary preference is no longer used here - the PostStepActionModal
-    // handles the decision of whether to save to library, use now, or both
+  const handleFormSubmit = async (data: StepCreate) => {
     setIsSubmitting(true)
     setError(null)