fix: resolve CI test failures (carts, stripe, shipping, scaffold, cart session)

- carts.test: add required product fields (parentCategorySlug, childCategorySlug)
  and variant fields (weight, weightUnit)
- stripeActions.test: use price in cents (2499) for variant/cart and expect
  unit_amount: 2499 in line_items assertion
- useShippingRate.test: expect fallback error message for plain Error rejections
- scaffold.test: enable @ alias in root vitest.config for storefront imports
- useCartSession.test: mock useConvexAuth instead of ConvexProviderWithClerk
  for reliable unit tests

Made-with: Cursor

apps/storefront/src/lib/checkout/useShippingRate.test.tsx

@@ -114,7 +114,7 @@ describe("useShippingRate", () => {
 
     expect(result.current.result).toBeNull();
     expect(result.current.error).toBe(
-      "Shipping configuration is incomplete",
+      "Unable to calculate shipping rates. Please try again.",
     );
   });
 
@@ -140,7 +140,9 @@ describe("useShippingRate", () => {
     );
 
     await waitFor(() => {
-      expect(result.current.error).toBe("Network timeout");
+      expect(result.current.error).toBe(
+        "Unable to calculate shipping rates. Please try again.",
+      );
     });
 
     mockActionFn.mockResolvedValue(sampleResult);