Remove empty columns from grid zones

Columns with zero cells (e.g. from tertiary detection where the word
was assigned to a neighboring column by overlap) are stripped from the
final result. Remaining columns and cells are re-indexed.

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

klausur-service/backend/grid_editor_api.py

@@ -1731,6 +1731,27 @@ async def _build_grid_core(
                 if fixed != text:
                     cell["text"] = fixed
 
+    # --- Remove empty columns (no cells assigned) ---
+    for z in zones_data:
+        cells = z.get("cells", [])
+        used_col_indices = {c.get("col_index") for c in cells}
+        old_cols = z.get("columns", [])
+        new_cols = [c for c in old_cols if c.get("col_index", c.get("index", -1)) in used_col_indices]
+        if len(new_cols) < len(old_cols):
+            # Re-index columns and cells
+            old_to_new = {}
+            for new_i, col in enumerate(new_cols):
+                old_i = col.get("col_index", col.get("index", new_i))
+                old_to_new[old_i] = new_i
+                col["col_index"] = new_i
+                col["index"] = new_i
+                col["label"] = f"column_{new_i + 1}" if len(new_cols) > 1 else "column_text"
+            for cell in cells:
+                old_ci = cell.get("col_index", 0)
+                cell["col_index"] = old_to_new.get(old_ci, old_ci)
+                cell["col_type"] = f"column_{cell['col_index'] + 1}" if len(new_cols) > 1 else "column_text"
+            z["columns"] = new_cols
+
     # Clean up internal flags before returning
     for z in zones_data:
         for cell in z.get("cells", []):