Checkout: remove vestigial HTLC-era references from the CCTP crypto flow

[0xdevcollins]

Rescoped — the CRITICAL fund-loss bug no longer exists

This issue originally described hardcoded zero-address HTLC params in CryptoPayment.tsx sending funds to the burn address. That code is gone — the crypto checkout has since been rebuilt on Circle CCTP V2:

• CryptoPayment.tsx calls useCryptoSelect → POST /v1/checkout/:paymentId/select-crypto, which returns server-generated wallet.approve and wallet.burn calldata (the API is the only place that knows the CCTP V2 ABI).
• The client signs approve(USDC, TokenMessengerV2, amount) then the burn using the addresses/calldata the server provides. Nothing is hardcoded; there is no client-side HTLC param construction.
• Verified (2026-07): no 0x0000… zero addresses and no htlcAddress / hashlock / timelock construction remain anywhere in apps/checkout.

The fund-loss vulnerability is resolved. Downgraded from CRITICAL/security to a minor cleanup.

Remaining work (minor)

Vestigial HTLC-era references linger and should be purged so the UI matches the CCTP model:

• apps/checkout/components/ErrorCard.tsx — an HTLC_TIMEOUT error entry
• apps/checkout/components/ConfirmPageClient.tsx — treats HTLC_TIMEOUT as a terminal status

Replace these with the CCTP flow's real failure modes (attestation timeout, quote expired), then do a one-off end-to-end pass of the crypto checkout on testnet (approve → burn → settle) to confirm.

Acceptance

• No HTLC_* strings remain in apps/checkout.
• Crypto checkout approve → burn → settle verified once on testnet.

[0xdevcollins]

Re-scoped: crypto checkout stays in v1; the HTLC-params framing is replaced by escrow intent params from the API (docs/10-api-design-spec.md).