Product Costs: Enter Once, Used Everywhere

Unit Cost, Landed Cost, and Dropship Unit Cost are now three clear, distinct fields on every product — editable from the product page, the Costs product sheet, or CSV import. Purchase order lines and dropship fulfillment lines default to the right cost automatically, and margin uses Landed Cost with Unit Cost as a fallback, so an incomplete cost is never shown as profitability. All three carry their full precision everywhere they're shown, including the product sheet.