Fix TypeError: unit_price.toFixed is not a function in SupplierProductManager

- Convert unit_price to Number before calling toFixed() on line 239
- Handles cases where unit_price may be a string, null, or undefined
- Uses fallback to 0 for invalid values to prevent runtime errors

This fixes the error that occurred when displaying supplier product prices in the onboarding supplier step.

frontend/src/components/domain/setup-wizard/steps/SupplierProductManager.tsx

@@ -236,7 +236,7 @@ export const SupplierProductManager: React.FC<SupplierProductManagerProps> = ({
                   {getProductName(priceList.inventory_product_id)}
                 </span>
                 <span className="text-[var(--text-secondary)] ml-2">
-                  €{priceList.unit_price.toFixed(2)}/{priceList.unit_of_measure}
+                  €{Number(priceList.unit_price || 0).toFixed(2)}/{priceList.unit_of_measure}
                 </span>
                 {priceList.minimum_order_quantity && priceList.minimum_order_quantity > 1 && (
                   <span className="text-xs text-[var(--text-secondary)] ml-2">