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/api/steps.ts

@@ -45,7 +45,7 @@ export const stepsApi = {
   },
 
   async getPopularTags(): Promise<PopularTag[]> {
-    const response = await apiClient.get<PopularTag[]>('/steps/popular-tags')
+    const response = await apiClient.get<PopularTag[]>('/steps/tags/popular')
     return response.data
   },