Fall back to the collection account when a personal IBAN cannot be issued

[TaprootFreak]

What

When a personal IBAN cannot be issued for an eligible bank-transfer customer (for example while the personal-IBAN provider is temporarily unavailable), the buy flow now shows the shared collection account with the per-buy reference instead of failing the request outright.

Previously this case threw PersonalIbanIssuanceFailed, leaving an eligible customer with no way to pay in during an outage.

Rules

• Only on real issuance failure for an eligible customer. A customer below KYC 50 still gets KycRequired; an unsupported currency still gets PersonalIbanCurrencyNotSupported. The personal-IBAN / KYC coupling is unchanged.
• Only with a reference. The collection account is shared, so the incoming transfer is attributed through the per-buy reference. Without a reference the transfer cannot be matched to a customer, so it is never shown — the request still fails in that case.
• Always logged at ERROR. The customer no longer sees an error, so the failure is logged at ERROR to keep a provider outage visible in monitoring.
• Both the explicit-provider path and the user-level path share one collectionAccountOrThrow helper, so they degrade identically.

Files

• buy.service.ts — resolveBankInfo (explicit + user-level paths) and the new collectionAccountOrThrow helper; DfxLogger added.
• buy.service.spec.ts — new cases (fallback with reference + ERROR log; no-reference still fails; explicit-provider fallback) and existing issuance-failure cases updated for the new getBank call.

Verification note

tsc and Prettier are clean on the changed files. The full buy.service.spec suite could not be run to green locally (the build host stalled on this large suite); the test changes are derived by inspection and rely on CI here for execution — please treat the CI test result as the gate.