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>

admin-lehrer/components/grid-editor/GridTable.tsx

@@ -488,7 +488,13 @@ export function GridTable({
                       } ${isMultiSelected ? 'bg-teal-50/60 dark:bg-teal-900/20' : ''} ${
                         isLowConf && !isMultiSelected ? 'bg-amber-50/50 dark:bg-amber-900/10' : ''
                       } ${row.is_header && !isMultiSelected ? 'bg-blue-50/50 dark:bg-blue-900/10' : ''}`}
-                      style={{ height: `${rowH}px` }}
+                      style={{
+                        height: `${rowH}px`,
+                        ...(cell?.box_region?.bg_hex ? {
+                          backgroundColor: `${cell.box_region.bg_hex}12`,
+                          borderLeft: cell.box_region.border ? `3px solid ${cell.box_region.bg_hex}60` : undefined,
+                        } : {}),
+                      }}
                       onContextMenu={(e) => {
                         if (onSetCellColor) {
                           e.preventDefault()