feat: Implement page-split step with auto-detection and sub-session naming

StepPageSplit now:
- Auto-calls POST /page-split on step entry
- Shows oriented image + detection result
- If double page: creates sub-sessions named "Title — S. 1/2"
- If single page: green badge "keine Trennung noetig"
- Manual "Weiter" button (no auto-advance)

Also:
- StepOrientation wrapper simplified (no page-split in orientation)
- StepUpload passes name back via onUploaded(sid, name)
- page.tsx: after page-split "Weiter" switches to first sub-session
- useKombiPipeline exposes setSessionName

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

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

@@ -4,17 +4,17 @@ import { StepOrientation as BaseStepOrientation } from '@/components/ocr-pipelin
 
 interface StepOrientationProps {
   sessionId: string | null
-  onNext: (sessionId: string) => void
+  onNext: () => void
   onSessionList: () => void
 }
 
-/** Thin wrapper around the shared StepOrientation component */
+/** Thin wrapper — adapts the shared StepOrientation to the Kombi pipeline's simpler onNext() */
 export function StepOrientation({ sessionId, onNext, onSessionList }: StepOrientationProps) {
   return (
     <BaseStepOrientation
       key={sessionId}
       sessionId={sessionId}
-      onNext={onNext}
+      onNext={() => onNext()}
       onSessionList={onSessionList}
     />
   )