Use this link, not the file in Files. Files will not open this properly in Safari. Open this page in Safari, tap Share, then Add to Home Screen, then Add. It will then open like an app.
Shortcut: Shortcuts app, tap +, Add Action, search Open URLs, paste this page link, name it Back Workout, tap Done.
This saves on this phone only, no account, no internet needed after you save it. A fresh hosted page starts with a fresh streak, because Safari saves progress per site, not per file.