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:

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

@@ -1,28 +1,52 @@
 'use client'
 
-import { useState, useCallback } from 'react'
+import { useState, useCallback, useEffect } from 'react'
 import { DOCUMENT_CATEGORIES, type DocumentCategory } from '@/app/(admin)/ai/ocr-pipeline/types'
 
 const KLAUSUR_API = '/klausur-api'
 
 interface StepUploadProps {
+  sessionId: string | null
   onUploaded: (sessionId: string) => void
+  onNext: () => void
 }
 
-export function StepUpload({ onUploaded }: StepUploadProps) {
+export function StepUpload({ sessionId, onUploaded, onNext }: StepUploadProps) {
   const [dragging, setDragging] = useState(false)
   const [uploading, setUploading] = useState(false)
+  const [selectedFile, setSelectedFile] = useState<File | null>(null)
+  const [preview, setPreview] = useState<string | null>(null)
   const [title, setTitle] = useState('')
   const [category, setCategory] = useState<DocumentCategory>('vokabelseite')
   const [error, setError] = useState('')
 
-  const handleUpload = useCallback(async (file: File) => {
+  // Clean up preview URL on unmount
+  useEffect(() => {
+    return () => { if (preview) URL.revokeObjectURL(preview) }
+  }, [preview])
+
+  const handleFileSelect = useCallback((file: File) => {
+    setSelectedFile(file)
+    setError('')
+    if (file.type.startsWith('image/')) {
+      setPreview(URL.createObjectURL(file))
+    } else {
+      setPreview(null)
+    }
+    // Auto-fill title from filename if empty
+    if (!title.trim()) {
+      setTitle(file.name.replace(/\.[^.]+$/, ''))
+    }
+  }, [title])
+
+  const handleUpload = useCallback(async () => {
+    if (!selectedFile) return
     setUploading(true)
     setError('')
 
     try {
       const formData = new FormData()
-      formData.append('file', file)
+      formData.append('file', selectedFile)
       if (title.trim()) formData.append('name', title.trim())
 
       const res = await fetch(`${KLAUSUR_API}/api/v1/ocr-pipeline/sessions`, {
@@ -53,20 +77,158 @@ export function StepUpload({ onUploaded }: StepUploadProps) {
     } finally {
       setUploading(false)
     }
-  }, [title, category, onUploaded])
+  }, [selectedFile, title, category, onUploaded])
 
   const handleDrop = useCallback((e: React.DragEvent) => {
     e.preventDefault()
     setDragging(false)
     const file = e.dataTransfer.files[0]
-    if (file) handleUpload(file)
-  }, [handleUpload])
+    if (file) handleFileSelect(file)
+  }, [handleFileSelect])
 
-  const handleFileSelect = useCallback((e: React.ChangeEvent<HTMLInputElement>) => {
+  const handleInputChange = useCallback((e: React.ChangeEvent<HTMLInputElement>) => {
     const file = e.target.files?.[0]
-    if (file) handleUpload(file)
-  }, [handleUpload])
+    if (file) handleFileSelect(file)
+  }, [handleFileSelect])
 
+  const clearFile = useCallback(() => {
+    setSelectedFile(null)
+    if (preview) URL.revokeObjectURL(preview)
+    setPreview(null)
+  }, [preview])
+
+  // ---- Phase 2: Uploaded → show result + "Weiter" ----
+  if (sessionId) {
+    return (
+      <div className="space-y-4">
+        <div className="bg-green-50 dark:bg-green-900/20 border border-green-200 dark:border-green-800 rounded-lg p-4">
+          <div className="flex items-center gap-2 text-green-700 dark:text-green-300 text-sm font-medium mb-3">
+            <span>&#10003;</span> Dokument hochgeladen
+          </div>
+          <div className="flex gap-4">
+            <div className="w-48 h-64 rounded-lg overflow-hidden bg-gray-100 dark:bg-gray-700 flex-shrink-0 border border-gray-200 dark:border-gray-600">
+              {/* eslint-disable-next-line @next/next/no-img-element */}
+              <img
+                src={`${KLAUSUR_API}/api/v1/ocr-pipeline/sessions/${sessionId}/image`}
+                alt="Hochgeladenes Dokument"
+                className="w-full h-full object-contain"
+                onError={(e) => { (e.target as HTMLImageElement).style.display = 'none' }}
+              />
+            </div>
+            <div className="text-sm text-gray-600 dark:text-gray-400">
+              <div className="font-medium text-gray-700 dark:text-gray-300 mb-1">
+                {title || 'Dokument'}
+              </div>
+              <div className="text-xs text-gray-400 mt-1">
+                Kategorie: {DOCUMENT_CATEGORIES.find(c => c.value === category)?.label || category}
+              </div>
+              <div className="text-xs font-mono text-gray-400 mt-1">
+                Session: {sessionId.slice(0, 8)}...
+              </div>
+            </div>
+          </div>
+        </div>
+
+        <div className="flex justify-end">
+          <button
+            onClick={onNext}
+            className="px-6 py-2.5 bg-teal-600 text-white text-sm font-medium rounded-lg hover:bg-teal-700 transition-colors"
+          >
+            Weiter &rarr;
+          </button>
+        </div>
+      </div>
+    )
+  }
+
+  // ---- Phase 1b: File selected → preview + "Hochladen" ----
+  if (selectedFile) {
+    return (
+      <div className="space-y-4">
+        {/* Title input */}
+        <div>
+          <label className="block text-sm font-medium text-gray-700 dark:text-gray-300 mb-1">
+            Titel
+          </label>
+          <input
+            type="text"
+            value={title}
+            onChange={(e) => setTitle(e.target.value)}
+            placeholder="z.B. Vokabeln Unit 3"
+            className="w-full px-3 py-2 border border-gray-300 dark:border-gray-600 rounded-lg bg-white dark:bg-gray-800 text-sm"
+          />
+        </div>
+
+        {/* Category selector */}
+        <div>
+          <label className="block text-sm font-medium text-gray-700 dark:text-gray-300 mb-1">
+            Kategorie
+          </label>
+          <div className="grid grid-cols-4 gap-1.5">
+            {DOCUMENT_CATEGORIES.map(cat => (
+              <button
+                key={cat.value}
+                onClick={() => setCategory(cat.value)}
+                className={`text-xs px-2 py-1.5 rounded-md text-left transition-colors ${
+                  category === cat.value
+                    ? 'bg-teal-100 dark:bg-teal-900/40 text-teal-700 dark:text-teal-300 ring-1 ring-teal-400'
+                    : 'bg-gray-50 dark:bg-gray-700 text-gray-600 dark:text-gray-400 hover:bg-gray-100'
+                }`}
+              >
+                {cat.icon} {cat.label}
+              </button>
+            ))}
+          </div>
+        </div>
+
+        {/* File preview */}
+        <div className="border border-gray-200 dark:border-gray-700 rounded-xl p-4">
+          <div className="flex items-start gap-4">
+            {preview ? (
+              <div className="w-36 h-48 rounded-lg overflow-hidden bg-gray-100 dark:bg-gray-700 flex-shrink-0 border border-gray-200 dark:border-gray-600">
+                {/* eslint-disable-next-line @next/next/no-img-element */}
+                <img src={preview} alt="Vorschau" className="w-full h-full object-contain" />
+              </div>
+            ) : (
+              <div className="w-36 h-48 rounded-lg bg-gray-100 dark:bg-gray-700 flex-shrink-0 flex items-center justify-center border border-gray-200 dark:border-gray-600">
+                <span className="text-3xl">&#128196;</span>
+              </div>
+            )}
+            <div className="flex-1 min-w-0">
+              <div className="font-medium text-sm text-gray-700 dark:text-gray-300 truncate">
+                {selectedFile.name}
+              </div>
+              <div className="text-xs text-gray-400 mt-1">
+                {(selectedFile.size / 1024 / 1024).toFixed(1)} MB
+              </div>
+              <button
+                onClick={clearFile}
+                className="text-xs text-red-500 hover:text-red-700 mt-2"
+              >
+                Andere Datei waehlen
+              </button>
+            </div>
+          </div>
+
+          <button
+            onClick={handleUpload}
+            disabled={uploading}
+            className="mt-4 w-full px-4 py-2.5 bg-teal-600 text-white text-sm font-medium rounded-lg hover:bg-teal-700 disabled:opacity-50 disabled:cursor-not-allowed transition-colors"
+          >
+            {uploading ? 'Wird hochgeladen...' : 'Hochladen'}
+          </button>
+        </div>
+
+        {error && (
+          <div className="text-sm text-red-500 bg-red-50 dark:bg-red-900/20 p-3 rounded-lg">
+            {error}
+          </div>
+        )}
+      </div>
+    )
+  }
+
+  // ---- Phase 1a: No file → drop zone ----
   return (
     <div className="space-y-4">
       {/* Title input */}
@@ -116,25 +278,19 @@ export function StepUpload({ onUploaded }: StepUploadProps) {
             : 'border-gray-300 dark:border-gray-600 hover:border-gray-400'
         }`}
       >
-        {uploading ? (
-          <div className="text-sm text-gray-500">Wird hochgeladen...</div>
-        ) : (
-          <>
-            <div className="text-4xl mb-3">📤</div>
-            <div className="text-sm text-gray-600 dark:text-gray-400 mb-2">
-              Bild oder PDF hierher ziehen
-            </div>
-            <label className="inline-block px-4 py-2 bg-teal-600 text-white text-sm rounded-lg cursor-pointer hover:bg-teal-700">
-              Datei auswaehlen
-              <input
-                type="file"
-                accept="image/*,.pdf"
-                onChange={handleFileSelect}
-                className="hidden"
-              />
-            </label>
-          </>
-        )}
+        <div className="text-4xl mb-3">&#128228;</div>
+        <div className="text-sm text-gray-600 dark:text-gray-400 mb-2">
+          Bild oder PDF hierher ziehen
+        </div>
+        <label className="inline-block px-4 py-2 bg-teal-600 text-white text-sm rounded-lg cursor-pointer hover:bg-teal-700">
+          Datei auswaehlen
+          <input
+            type="file"
+            accept="image/*,.pdf"
+            onChange={handleInputChange}
+            className="hidden"
+          />
+        </label>
       </div>
 
       {error && (