Pangolin is a reverse proxy, so it can forward a URL to any backend service on any port. But you’re right in that you have to be signed in on the browser you access it on. Therefore an app won’t directly work without prior login. You can create a ‘shareable link’ in Pangolin, which I use for the Immich app. This gives me header tokens that the Immich app can take in its advanced settings, and that’s how that one works.

I’ve recently moved away from dedicated apps for mobile services and toward web-based access for most things (I use Music Assistant in browser). This isn’t perfect for everything and everyone, but I realise now with your question that it’s worked well for me transitioning to Pangolin (and at least Immich app works).