Great news - my frontend can do the onboarding procedure! It successfully uploads my h-card and posts a “first post” to the blog, so it won’t be empty.
For now I didn’t do authentication yet. I’m still thinking if I should do password+TOTP auth, just a password or a WebAuthn. Or maybe I could just use indieauth.com directly, but that needs at least one social profile in the h-card, which is not guaranteed right now.