OptionalcompletionHost route that finalizes tokenize-first payments (Paysafe, PayPal). When
set, usePay/<PayButton> derive onServerCompletion automatically —
they POST { sessionRef, clientToken, billingDetails? } here (mount
@payfanout/server's createCompletionHandler at this path) and no
per-surface completion wiring is needed. An explicit onServerCompletion
still wins, as the escape hatch for a custom transport.
OptionalinitialDefaults to the first adapter.
OptionallocaleBCP-47 locale for the text the library renders itself — currently the
default <PayButton> label. PayFanout ships en/fr/de/es; unknown locales
fall back to English. Does not touch the PSP's hosted field texts (set
those via <PaymentFields locale>).
Client adapters only — no secrets, no server logic.