StepBoxGridReview: show box border color from structure detection

- Use box_bg_hex for border color (from Step 7 structure detection)
- Numbered color badges per box
- Show color name in box header
- Add box_bg_color/box_bg_hex to GridZone type

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

admin-lehrer/components/grid-editor/types.ts

@@ -75,6 +75,8 @@ export interface GridZone {
   vsplit_group?: number
   box_layout_type?: 'flowing' | 'columnar' | 'bullet_list' | 'header_only'
   box_grid_reviewed?: boolean
+  box_bg_color?: string
+  box_bg_hex?: string
 }
 
 export interface BBox {