Signup — Lu.ma-style one-page flow

Coaches landing on orbi.so/signup now get a single one-page flow inspired by Lu.ma:

• One entry point: email or Google. The old /signup/coach and /signup/client URLs still work — they redirect to /signup so any link out in the wild keeps resolving.
• Six-digit code by email (no password to invent up front). Or one-tap Google in a popup so the parent page stays alive and slides into the next step.
• After verification, you pick your practice name and your booking-page URL (orbi.so/book/<slug>) right there, in the same view.
• A soft sunrise animation in the background hints at the booking page that will be yours.

Under the hood:

• New claim_coach_role(p_user_id, p_name, p_slug) RPC stays service-role-only — same security model as the hardened version, just with an optional slug so coaches choose their URL during onboarding instead of getting one auto-derived from their name.
• OAuth callbacks support a popup mode that postMessages back to the parent and closes itself, so the practice-setup step picks up exactly where the email step left off.
• The unsolicited bodybuilder marketing image is gone. Auth pages are a single centred column now.