Is JetFuelX integrated directly into my ForeFlight Mobile app, or ForeFlight on the web?

No, JetFuelX is a standalone website. However, if you have a ForeFlight account you can sign in to the JetFuelX website using your ForeFlight username and password. Beginning with ForeFlight version 9.1, JetFuelX fuel prices can be seen in the ForeFlight Mobile app when you have linked JetFuelX with ForeFlight and have a Performance Plus, Business Performance, or MFB Performance subscription plan. For more details please see this article.