- Add new POST API endpoints at `/api/v1/stock-purchase-items/[id]/sku` and `/api/v1/stock-purchase-items/[id]/cost` for updating stock purchase item details
- Update ItemsTable to support cost and SKU edits for all row types, add supplier SKU column and editing for stock rows
- Extend PoCard to pass edit callbacks and add stock-specific invoice upload functionality
- Update all related test suites to cover new editing and UI features
- Adjust invoice upload messaging to differentiate between customer and stock purchase orders
