feat: Add custom step continuation flow with save/use/branch options

Custom steps during tree navigation now support a complete workflow:
- PostStepActionModal: Save for Later / Use Now / Both options
- ContinuationModal: Pick descendant nodes or build custom branch
- ForkTreeModal: Save modified tree as personal copy at completion
- Custom steps are recorded in decisions array for export
- Fix popular-tags API endpoint URL mismatch
- Add aria-labels for accessibility on select/button elements

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

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

@@ -340,6 +340,7 @@ export function StepForm({ onSubmit, onCancel, initialData }: StepFormProps) {
                   type="button"
                   onClick={() => removeTag(tag)}
                   className="rounded-full hover:bg-primary/20"
+                  aria-label={`Remove tag ${tag}`}
                 >
                   <X className="h-3 w-3" />
                 </button>