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/services/catalogueApi.ts

@@ -103,7 +103,7 @@ async function fallbackCreateProductLocation(items: ImportCatalogueProductReques
     tenantid: item.tenantid,
     locationid: item.locationid,
     productid: item.catalogueid,
-    quantity: item.quantity || 1,
+    quantity: item.quantity ?? 0,
     stocktype: item.stocktype || "in",
     status: item.status || "Active",
     retailprice: item.retailprice || 0,