fix: useMemo vor Early Returns verschieben (Rules of Hooks)

boxZonesPct useMemo war nach bedingten Returns platziert, was gegen
Reacts Rules of Hooks verstoesst und einen Client-Side Crash ausloest.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

admin-lehrer/components/ocr-pipeline/StepReconstruction.tsx

@@ -427,6 +427,20 @@ export function StepReconstruction({ sessionId, onNext }: StepReconstructionProp
     return Math.max(8, Math.min(18, px * (zoom / 100)))
   }, [imageNaturalH, zoom])
 
+  // Box zones in percent for clamping cell positions in overlay mode
+  const boxZonesPct = useMemo(() =>
+    parentZones
+      .filter(z => z.zone_type === 'box' && z.box)
+      .map(z => {
+        const imgH = imageNaturalSize?.h || 1
+        return {
+          topPct: (z.box!.y / imgH) * 100,
+          bottomPct: ((z.box!.y + z.box!.height) / imgH) * 100,
+        }
+      }),
+    [parentZones, imageNaturalSize]
+  )
+
   if (!sessionId) {
     return <div className="text-center py-12 text-gray-400">Bitte zuerst eine Session auswaehlen.</div>
   }
@@ -476,20 +490,6 @@ export function StepReconstruction({ sessionId, onNext }: StepReconstructionProp
     )
   }
 
-  // Box zones in percent for clamping cell positions in overlay mode
-  const boxZonesPct = useMemo(() =>
-    parentZones
-      .filter(z => z.zone_type === 'box' && z.box)
-      .map(z => {
-        const imgH = imageNaturalSize?.h || 1
-        return {
-          topPct: (z.box!.y / imgH) * 100,
-          bottomPct: ((z.box!.y + z.box!.height) / imgH) * 100,
-        }
-      }),
-    [parentZones, imageNaturalSize]
-  )
-
   // Clamp cell positions so they don't overlap with box zones
   const adjustCellForBoxZones = (
     bboxPct: { x: number; y: number; w: number; h: number },