fix: stop sending client-computed price on order creation

createOrder() sent a discount-applied price per line item that the
client computed itself, with no server revalidation. Items now only
carry productId/name/quantity - the backend must price from its own
catalog. createPayment()'s amount (required to actually charge the
payment gateway) is unchanged; backend must revalidate it instead,
tracked in BACKEND-API-REFERENCE.md §12.

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

src/app/services/api.service.ts

@@ -54,7 +54,12 @@ export interface CartPaymentRequest {
 }
 
 export interface CreateOrderRequest {
-  items: Array<{ productId: string; name: string; quantity: number; price: number }>;
+  /**
+   * No `price` field: the backend must price each line item from its own
+   * catalog by `productId`, never trust a client-supplied amount.
+   * See BACKEND-API-REFERENCE.md §12.
+   */
+  items: Array<{ productId: string; name: string; quantity: number }>;
   customer: { name: string; email: string; phone: string };
   payment?: { method: string; currency: string };
   shipping?: { address: string; method: string; trackingNumber: string };