fix: use cart_url() instead of url_for('cart.view_cart') for cross-app redirect

The market app doesn't have a cart blueprint registered, so url_for()
would fail. Uses the cross-app cart_url() helper instead.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

bp/product/routes.py

@@ -242,7 +242,8 @@ def register():
             )
 
         # normal POST: go to cart page
-        return redirect(url_for("cart.view_cart"))
+        from shared.urls import cart_url
+        return redirect(cart_url("/"))