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/app/(admin)/ai/ocr-kombi/page.tsx

@@ -16,6 +16,7 @@ import { StepStructure } from '@/components/ocr-kombi/StepStructure'
 import { StepGridBuild } from '@/components/ocr-kombi/StepGridBuild'
 import { StepGridReview } from '@/components/ocr-kombi/StepGridReview'
 import { StepGutterRepair } from '@/components/ocr-kombi/StepGutterRepair'
+import { StepBoxGridReview } from '@/components/ocr-kombi/StepBoxGridReview'
 import { StepGroundTruth } from '@/components/ocr-kombi/StepGroundTruth'
 import { useKombiPipeline } from './useKombiPipeline'
 
@@ -97,6 +98,8 @@ function OcrKombiContent() {
       case 10:
         return <StepGutterRepair sessionId={sessionId} onNext={handleNext} />
       case 11:
+        return <StepBoxGridReview sessionId={sessionId} onNext={handleNext} />
+      case 12:
         return (
           <StepGroundTruth
             sessionId={sessionId}