Transferred-in reg's ticket still offers an invoice/amount-due for the new event

[maebeale]

Follow-up from #1944 / PR #2044.

A transferred-in registration's payment lives on its source registration, so the registrant owes the new event nothing. But the registrant-facing ticket still treats it like a normal paid registration: EventRegistration#invoice_available? is event.cost_cents.to_i.positive?, so on a paid event the ticket can show an invoice / amount due for the new event — misleading, since there's nothing to pay here.

Suggested fix

• Suppress the invoice (and any "amount due" prompt) on a transferred-in reg's ticket, or point it at the source registration's invoice instead.
• Decide what the transferred-in ticket should show for payment (likely a "transferred in — see original registration" note).
• Re-check receipt_available? / w9_available? for consistency (currently fine: no payment on the reg → no receipt/W-9).

Context

• app/models/event_registration.rb — invoice_available?, receipt_available?, w9_available?.
• Ticket views under app/views/event_registrations/ (_ticket.html.erb) and app/views/events/callouts/.
• The admin edit form already shows a read-only "financials on the original registration" summary; this is the registrant-facing counterpart.