It's kind of unclear to me, can I use shuttle without using shuttle.rs (the platform) to actually run it?

Not that I am against paying for a service, but the idea of writing my app against with a specific library against a specific platform makes me uneasy.

They have a github project but I think that is just the CLI + rust libs?

First login failed with: "Callback handler failed. CAUSE: Missing state cookie from login request (check login URL, callback URL and cookie config)." but after retrying it went to Projects list. API Key copy button doesn't do anything.