Is there an iPhone or iPad app?

Yes. ScriptFox is on the App Store (listed as "Scriptfox") for iPhone and iPad; use the link in the footer of getscriptfox.com or search the App Store. It's a free download that uses the same account and scripts as the web. Sign in with "Continue with Apple", "Continue with Google", or email and password. The app follows your account's plan: subscribe on the website (account menu → "Plan & billing") and the phone sees it when you sign in. If a plan was bought inside the app through the App Store, where Pro costs $24.99 a month because Apple keeps a share, change or cancel it in your App Store subscriptions; the website's Plan & billing page shows it. There is an Android app too, on Google Play.