Unified Grid: merge all zones into single Excel-like grid

Backend (unified_grid.py):
- build_unified_grid(): merges content + box zones into one zone
- Dominant row height from median of content row spacings
- Full-width boxes: rows integrated directly
- Partial-width boxes: extra rows inserted when box has more text
  lines than standard rows fit (e.g., 7 lines in 5-row height)
- Box-origin cells tagged with source_zone_type + box_region metadata

Backend (grid_editor_api.py):
- POST /sessions/{id}/build-unified-grid → persists as unified_grid_result
- GET /sessions/{id}/unified-grid → retrieve persisted result

Frontend:
- GridEditorCell: added source_zone_type, box_region fields
- GridTable: box-origin cells get tinted background + left border
- StepAnsicht: split-view with original image (left) + editable
  unified GridTable (right). Auto-builds on first load.

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

klausur-service/backend/grid_editor_api.py

@@ -2350,3 +2350,62 @@ async def build_box_grids(session_id: str, request: Request):
         "spell_fixes": spell_fixes,
         "zones": zones,
     }
+
+
+# ---------------------------------------------------------------------------
+# Unified Grid endpoint
+# ---------------------------------------------------------------------------
+
+@router.post("/sessions/{session_id}/build-unified-grid")
+async def build_unified_grid_endpoint(session_id: str):
+    """Build a single-zone unified grid merging content + box zones.
+
+    Takes the existing multi-zone grid_editor_result and produces a
+    unified grid where boxes are integrated into the main row sequence.
+    Persists as unified_grid_result (preserves original multi-zone data).
+    """
+    session = await get_session_db(session_id)
+    if not session:
+        raise HTTPException(status_code=404, detail=f"Session {session_id} not found")
+
+    grid_data = session.get("grid_editor_result")
+    if not grid_data:
+        raise HTTPException(status_code=400, detail="No grid data. Run build-grid first.")
+
+    from unified_grid import build_unified_grid
+
+    result = build_unified_grid(
+        zones=grid_data.get("zones", []),
+        image_width=grid_data.get("image_width", 0),
+        image_height=grid_data.get("image_height", 0),
+        layout_metrics=grid_data.get("layout_metrics", {}),
+    )
+
+    # Persist as separate field (don't overwrite original multi-zone grid)
+    await update_session_db(session_id, unified_grid_result=result)
+
+    logger.info(
+        "build-unified-grid session %s: %d rows, %d cells",
+        session_id,
+        result.get("summary", {}).get("total_rows", 0),
+        result.get("summary", {}).get("total_cells", 0),
+    )
+
+    return result
+
+
+@router.get("/sessions/{session_id}/unified-grid")
+async def get_unified_grid(session_id: str):
+    """Retrieve the unified grid for a session."""
+    session = await get_session_db(session_id)
+    if not session:
+        raise HTTPException(status_code=404, detail=f"Session {session_id} not found")
+
+    result = session.get("unified_grid_result")
+    if not result:
+        raise HTTPException(
+            status_code=404,
+            detail="No unified grid. Run build-unified-grid first.",
+        )
+
+    return result