refactor: delete dead legacy payment code from ApiService

Verified zero remaining callers for each before deleting (grepped
src/app for every method name individually), not assumed from the
earlier commit's dead-code note.

Deleted from api.service.ts:
- createPayment() - legacy direct QR creation (POST {qrBaseUrl}/qr)
- createCartPayment() - legacy /cart payment creation, client-sent amount
- createPaymentIntent() - superseded by @marketplaces/payment's gateway
- checkCartPaymentStatus(), checkCartCardPaymentStatus(), checkPaymentStatus()
  - legacy QR/card status polls, superseded by the same gateway
- resolvePaymentQrId/resolvePaymentQrUrl/resolvePaymentLink/
  resolveBankPaymentUrl - QrCreateResponse field-normalization helpers,
  no longer had a caller once the methods above were gone
- Types: QrCreateRequest, QrCreateResponse, CartPaymentRequest,
  PaymentIntentRequest, QrDynamicStatusResponse
- Fields: qrBaseUrl, cartPaymentPartnerId - no longer read by anything
- Imports: HttpHeaders, environment - no longer used in this file

Did NOT touch createOrder() or createCheckoutSession() - both still have
live callers in cart.component.ts, confirmed before deciding what to keep.

cart.component.ts: corrected the createPaymentIntent() comment, which
referenced the deleted method/helper names, to name what actually got
deleted instead of what was merely "dead as of that commit."

docs/backend/FRONTEND-API-SURFACE-COMPLETE.md: moved the 4 now-deleted
QR/card endpoints out of the "still live" legacy table into a dated removal
note - the doc's own premise is "every endpoint this codebase currently
calls," so it was wrong to leave them listed as called once they weren't.
Legacy-undocumented count corrected 15->11, total 97->93.

Verified: production build succeeds (no dangling references), 247/247 unit
tests, arch:check clean.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/app/pages/cart/cart.component.ts

@@ -333,12 +333,12 @@ export class CartComponent implements OnDestroy {
   /**
    * Superseded api.service.ts's createPaymentIntent (POST
    * /api/v2/storefront/payments/intents, our own inferred contract) with
-   * @marketplaces/payment's real, published one. That method and the
+   * @marketplaces/payment's real, published one. That method, createPayment
+   * (legacy /qr), createCartPayment (legacy /cart), checkCartPaymentStatus,
+   * checkCartCardPaymentStatus, checkPaymentStatus, and the
    * QrCreateResponse-based resolvePaymentQrId/resolvePaymentQrUrl/
-   * resolvePaymentLink/resolveBankPaymentUrl helpers on ApiService are dead
-   * code as of this change - left in place rather than deleted in the same
-   * pass that adds a new external dependency, so a revert doesn't also need
-   * to resurrect deleted code.
+   * resolvePaymentLink/resolveBankPaymentUrl helpers - all now deleted from
+   * ApiService, confirmed dead first (zero remaining callers) before removal.
    */
   private createPaymentIntent(
     session: import('../../services/api.service').CheckoutSessionResponse,