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/StepUpload.tsx

@@ -7,7 +7,7 @@ const KLAUSUR_API = '/klausur-api'
 
 interface StepUploadProps {
   sessionId: string | null
-  onUploaded: (sessionId: string) => void
+  onUploaded: (sessionId: string, name: string) => void
   onNext: () => void
 }
 
@@ -71,7 +71,7 @@ export function StepUpload({ sessionId, onUploaded, onNext }: StepUploadProps) {
         })
       }
 
-      onUploaded(sid)
+      onUploaded(sid, title.trim() || selectedFile.name)
     } catch (e) {
       setError(e instanceof Error ? e.message : String(e))
     } finally {