Fix product import defaulting to 1 in stock, allowing orders with zero real inventory

Quick-import buttons hardcoded quantity:1, which the backend writes straight
to the stock ledger as an "in" entry — letting one order go through for a
product that was never actually stocked. Default to 0 instead, and fix a
fallback path that was silently coercing an explicit 0 back to 1 via `|| 1`.

src/components/ImportProductModal.tsx

@@ -143,7 +143,7 @@ export default function ImportProductModal({
                     catalogueid: product.id,
                     categoryid: validCategory,
                     subcategoryid: 0,
-                    quantity: 1,
+                    quantity: 0,
                     stocktype: "in",
                     status: "Draft",
                     retailprice: 0,