Keep page references as visible column cells

Step 5g was extracting page refs (p.55, p.70) as zone metadata and
removing them from the cell table. Users want to see them as a
separate column. Now keeps cells in place while still extracting
metadata for the frontend header display.

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

klausur-service/backend/grid_editor_api.py

@@ -1176,10 +1176,10 @@ async def _build_grid_core(
             })
             page_ref_cell_ids.add(cell.get("cell_id"))
 
-        # Remove page_ref cells from the table (but keep their rows)
-        if page_ref_cell_ids:
-            z["cells"] = [c for c in z["cells"]
-                          if c.get("cell_id") not in page_ref_cell_ids]
+        # Keep page_ref cells in the table as a visible column.
+        # Previously these were removed, but users want to see them.
+        # The metadata extraction above still populates zone["page_refs"]
+        # for the frontend header display.
 
         # Detect footer: last non-header row if it has only 1 cell
         # with short, non-content text (page numbers like "233" or