fix(shop): restore onSuccess for add-to-cart so new line appears

Add-to-cart can't optimistically render the new line item (onMutate
only has the variant ID, not the full product snapshot). Without
onSuccess setting the server response, the drawer opened empty until
the background refetch landed. Restore onSuccess for add only — the
concurrent-flicker risk is minimal here since rapid-fire adds are rare,
and the worst case is a brief badge-count wobble.

Author: tannerlinsley

src/hooks/useCart.ts

@@ -95,9 +95,13 @@ export function useAddToCart() {
         qc.setQueryData(CART_QUERY_KEY, ctx.previous)
     },
 
-    // No onSuccess — don't overwrite the cache with the server response
-    // while other mutations may be in flight. settleWhenIdle will refetch
-    // from the server once all mutations have landed.
+    // Add-to-cart needs onSuccess to populate the new line item in the
+    // cache — onMutate can only bump totalQuantity since it doesn't have
+    // the full product data. Unlike remove/update, rapid-fire adds are
+    // rare, and the worst case is a brief badge-count fluctuation.
+    onSuccess: (cart) => {
+      qc.setQueryData(CART_QUERY_KEY, cart)
+    },
 
     onSettled: () => settleWhenIdle(qc),
   })