Smarter Pay Now / level-payment invoice flow for registration

Goal

Follow-up to #239 (closed). When a team must pay before they can register and clicks Pay Now, route them to the correct level-payment invoice intelligently, and set clear expectations that paying does not, by itself, complete registration in all cases.

Desired behavior

• If the team has exactly one level they can pay for, auto-advance straight to that invoice.
• If the team has more than one payable level, present the selection with the appropriate option pre-checked, and let the coach decide whether paying for more than one is appropriate.
• Because of how invoices work — an invoice can be generated, its public link shared, and paid by someone other than the coach — we cannot reliably deep-link back into the registration flow after payment. The UI must make clear that completing payment does not automatically register the team into an event; the team returns and submits registration explicitly (or is registered if space remains, per the #239 (closed) design).

Acceptance criteria

• One payable level → Pay Now goes directly to that invoice.
• Multiple payable levels → selection UI with the appropriate option pre-checked; coach confirms.
• Messaging makes clear that payment is not the same as a confirmed event registration; no reliance on a post-payment deep-link back into the registration flow.

Follows #239 (closed).