Add Box-Grid-Review step (Step 11) to OCR pipeline

New pipeline step between Gutter Repair and Ground Truth that processes
embedded boxes (grammar tips, exercises) independently from the main grid.

Backend:
- cv_box_layout.py: classify_box_layout() detects flowing/columnar/
  bullet_list/header_only layout types per box
- build_box_zone_grid(): layout-aware grid building (single-column for
  flowing text, independent columns for tabular content)
- POST /sessions/{id}/build-box-grids endpoint with SmartSpellChecker
- Layout type overridable per box via request body

Frontend:
- StepBoxGridReview.tsx: shows each box with cropped image + editable
  GridTable. Layout type dropdown per box. Auto-builds on first load.
- Auto-skip when no boxes detected on page
- Pipeline steps updated: 13 steps (0-12), Ground Truth moved to 12

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

admin-lehrer/components/grid-editor/types.ts

@@ -73,6 +73,8 @@ export interface GridZone {
   header_rows: number[]
   layout_hint?: 'left_of_vsplit' | 'right_of_vsplit' | 'middle_of_vsplit'
   vsplit_group?: number
+  box_layout_type?: 'flowing' | 'columnar' | 'bullet_list' | 'header_only'
+  box_grid_reviewed?: boolean
 }
 
 export interface BBox {