Ship a custom-HTML embed or a Headless Auth integration

We need a developer to get an existing daily quiz app running natively inside our Circle.so community (web + branded iOS/Android app), so logged-in members see it with no separate login required. The quiz (a single self-contained HTML/vanilla-JS file, no framework) and its backend (a Google Apps Script web app behind a Google Sheet, called via JSONP) are already built and working - this is an integration job, not a build-from-scratch. We've already confirmed with Circle that custom code executes inside the branded app's WebView with no CSP restrictions, and that identity has to come through Circle's Headless Auth API (server mints a member token; there's no cross-domain session/SSO). Work is scoped in two parts: a short paid spike (half a day) to confirm the code executes and exposes the logged-in member's identity inside Circle's post/custom-screen surfaces, followed by one to two days to package the real quiz and wire up token-verified identity so plays can't be spoofed. Looking for someone strong in vanilla JS who's comfortable reading unfamiliar third-party API docs and debugging inside mobile webviews - prior Circle.so experience is a bonus, not a requirement. Full written brief with everything we've already ruled out is ready to share on request.