New pipeline step between Crop and Columns that visualizes detected
document structure: boxes (line-based + shading), page zones, and
color regions. Shows original image on the left, annotated overlay
on the right.
Backend: POST /detect-structure endpoint + /image/structure-overlay
Frontend: StepStructureDetection component with zone/box/color details
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
