Fix infinite loop in StepBoxGridReview auto-build

Auto-build was triggering on every grid.zones.length change, which
happens on every rebuild (zone indices increment). Now uses a ref
to ensure auto-build fires only once. Also removed boxZones.length===0
condition that could trigger unnecessary builds.

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

admin-lehrer/components/ocr-kombi/StepBoxGridReview.tsx

@@ -1,6 +1,6 @@
 'use client'
 
-import { useCallback, useEffect, useState } from 'react'
+import { useCallback, useEffect, useRef, useState } from 'react'
 import { useGridEditor } from '@/components/grid-editor/useGridEditor'
 import type { GridZone } from '@/components/grid-editor/types'
 import { GridTable } from '@/components/grid-editor/GridTable'
@@ -96,15 +96,16 @@ export function StepBoxGridReview({ sessionId, onNext }: StepBoxGridReviewProps)
     await buildBoxGrids({ [String(boxIdx)]: layoutType })
   }, [buildBoxGrids])
 
-  // Auto-build on first load if box zones have no cells
+  // Auto-build once on first load if box zones have no cells
+  const autoBuildDone = useRef(false)
   useEffect(() => {
-    if (!grid || loading || building) return
-    const needsBuild = boxZones.length === 0 || boxZones.some(z => !z.cells || z.cells.length === 0)
-    // Only auto-build if we know there are boxes (check structure_result via a quick fetch)
+    if (!grid || loading || building || autoBuildDone.current) return
+    const needsBuild = boxZones.some(z => !z.cells || z.cells.length === 0)
     if (needsBuild && sessionId) {
+      autoBuildDone.current = true
       buildBoxGrids()
     }
-  }, [grid?.zones?.length, loading]) // eslint-disable-line react-hooks/exhaustive-deps
+  }, [grid, loading]) // eslint-disable-line react-hooks/exhaustive-deps
 
   if (loading) {
     return (