Show full Grid-Review in Ground Truth step + GT badge in session list

- StepGroundTruth now shows the split view (original image + table)
  so the user can verify the final result before marking as GT
- Backend session list now returns is_ground_truth flag
- SessionList shows amber "GT" badge for marked sessions

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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

@@ -298,7 +298,7 @@ function SessionRow({
         </div>
       </div>
 
-      {/* Category badge */}
+      {/* Category + GT badge */}
       <div className="flex flex-col gap-1 items-end flex-shrink-0" onClick={(e) => e.stopPropagation()}>
         <button
           onClick={onToggleCategory}
@@ -311,6 +311,11 @@ function SessionRow({
         >
           {catInfo ? `${catInfo.icon} ${catInfo.label}` : '+ Kategorie'}
         </button>
+        {session.is_ground_truth && (
+          <span className="text-[10px] px-1.5 py-0.5 rounded-full bg-amber-100 dark:bg-amber-900/30 border border-amber-300 dark:border-amber-700 text-amber-700 dark:text-amber-300" title="Ground Truth markiert">
+            GT
+          </span>
+        )}
       </div>
 
       {/* Actions */}