feat: ImageLayoutEditor, arrow-key nav, multi-select bold, wider columns

- New ImageLayoutEditor: SVG overlay on original scan with draggable
  column dividers, horizontal guidelines (margins/header/footer),
  double-click to add columns, x-button to delete
- GridTable: MIN_COL_WIDTH 40→80px for better readability
- Arrow up/down keys navigate between rows in the grid editor
- Ctrl+Click for multi-cell selection, Ctrl+B to toggle bold on selection
- getAdjacentCell works for cells that don't exist yet (new rows/cols)
- deleteColumn now merges x-boundaries correctly
- Session restore fix: grid_editor_result/structure_result in session GET
- Footer row 3-state cycle, auto-create cells for empty footer rows
- Grid save/build/GT-mark now advance current_step=11

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

klausur-service/backend/ocr_pipeline_regression.py

@@ -256,7 +256,7 @@ async def mark_ground_truth(
     # Merge into existing ground_truth JSONB
     gt = session.get("ground_truth") or {}
     gt["build_grid_reference"] = reference
-    await update_session_db(session_id, ground_truth=gt)
+    await update_session_db(session_id, ground_truth=gt, current_step=11)
 
     logger.info(
         "Ground truth marked for session %s: %d cells",