feat(api): add customer accounts with live summary and append-only ledger

One balance row per (user, kind, currency): available and frozen carry the
platform base currency, points carries none. Monetary and points rows use
separate partial unique indexes because a plain UNIQUE lets NULL repeat.

Balance changes go through transactional primitives that debits guard with a
conditional update, credits add atomically, and freeze/release move both sides
in one transaction after locking rows by primary key. Every change appends an
immutable entry holding its resulting balance. Registration and a migration
backfill create the zero rows; GET /api/me/stats is the only public surface and
no endpoint mutates a balance.

The mall buyer center and points page drop the USER_STATS fixture for the
shared contract; the fixture stays exported so the fixed-data adapter can still
serve the account domain as a rollback path.

Implements openspec change add-customer-accounts.

apps/mall/plugins/api.ts

@@ -9,6 +9,7 @@ import { createMockApi } from "~/mock/api";
  */
 type LiveDomain =
   | "auth"
+  | "account"
   | "catalog"
   | "currency"
   | "content"
@@ -27,6 +28,7 @@ type LiveDomain =
  */
 const LIVE_PICKS = {
   auth: (a: ApiClient) => ({ register: a.register, login: a.login, me: a.me }),
+  account: (a: ApiClient) => ({ getAccountSummary: a.getAccountSummary }),
   catalog: (a: ApiClient) => ({
     listProducts: a.listProducts,
     getProduct: a.getProduct,
@@ -76,6 +78,7 @@ const DEFAULT_LIVE_DOMAINS: LiveDomain[] = [
   "shops",
   "brands",
   "auth",
+  "account",
   "cart",
   "orders",
   "shipments",