fix: Redesign StepUpload for manual step control

StepUpload now has 3 phases:
1. File selection: drop zone / file picker → shows preview
2. Review: title input, category, file info → "Hochladen" button
3. Uploaded: shows session image → "Weiter" button

No more auto-advance after upload. User controls every step.
openSession() removed from onUploaded callback to prevent
step-reset race condition.

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

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

@@ -53,11 +53,12 @@ function OcrKombiContent() {
       case 0:
         return (
           <StepUpload
+            sessionId={sessionId}
             onUploaded={(sid) => {
               setSessionId(sid)
               loadSessions()
-              openSession(sid)
             }}
+            onNext={handleNext}
           />
         )
       case 1: