feat(mall): classic B2B2C PC storefront with fixed mock API layer

- Mock adapter implementing @vmall/shared ApiClient (localStorage-persisted
  cart/orders/invoices), runtime switch via mockApi flag (default on)
- Fixed bilingual mock catalog: 3-level categories, 24 products w/ SKUs,
  stores, brands, banners, floors, seckill/collective/integral, comments,
  coupons, addresses, seeded orders/shipments/invoices
- B2B2C mall shell: top bar, logo/search/cart header, dark nav + category
  mega-menu, value-prop footer, back-to-top; 1200px grid, #ca151e theme
- UI primitives replacing element-plus: carousel, pagination, breadcrumb,
  qty stepper, rating stars, modal, tabs, step bar, product card
- Pages: home floors, search (filters/sort/paging), goods detail (SKU
  picker, store rail, review tabs), cart -> checkout -> pay -> success,
  auth pages, user center (dashboard/orders/addresses/favorites/coupons/
  invoices), stores, seckill, collective, integral
- i18n split into per-domain locale modules (en+zh)
- OpenSpec change mall-pc-storefront-replica archived; all specs green

apps/mall/app.vue

@@ -1,56 +1,15 @@
 <script setup lang="ts">
 const session = useSessionStore();
-const { locale, locales, setLocale } = useI18n();
-const { currency } = usePrefs();
-const { $api } = useNuxtApp();
-
 onMounted(() => session.hydrate());
-
-const currencies = ref<string[]>(["USD"]);
-onMounted(async () => {
-  try {
-    const list = await $api.listCurrencies();
-    if (list.length > 0) currencies.value = list.map((c) => c.code);
-  } catch {
-    /* API may be down during SSR-less dev */
-  }
-});
 </script>
 
 <template>
   <div>
-    <header class="topnav">
-      <div class="container topnav-inner">
-        <NuxtLink to="/" class="brand">{{ $t("common.appName") }}</NuxtLink>
-        <nav>
-          <NuxtLink to="/">{{ $t("nav.home") }}</NuxtLink>
-          <NuxtLink to="/cart">{{ $t("nav.cart") }}</NuxtLink>
-          <NuxtLink to="/orders">{{ $t("nav.orders") }}</NuxtLink>
-          <NuxtLink to="/invoices">{{ $t("nav.invoices") }}</NuxtLink>
-        </nav>
-        <select
-          :value="currency"
-          :aria-label="$t('common.currency')"
-          @change="currency = ($event.target as HTMLSelectElement).value"
-        >
-          <option v-for="c in currencies" :key="c" :value="c">{{ c }}</option>
-        </select>
-        <select
-          :value="locale"
-          :aria-label="$t('common.language')"
-          @change="setLocale(($event.target as HTMLSelectElement).value as 'en' | 'zh')"
-        >
-          <option v-for="l in locales" :key="l.code" :value="l.code">{{ l.name }}</option>
-        </select>
-        <template v-if="session.isLoggedIn">
-          <span class="muted">{{ session.user?.display_name }}</span>
-          <button class="btn sm" @click="session.logout()">{{ $t("common.logout") }}</button>
-        </template>
-        <NuxtLink v-else to="/login" class="btn sm primary">{{ $t("common.login") }}</NuxtLink>
-      </div>
-    </header>
-    <main class="container page">
+    <ShellTopBar />
+    <ShellSiteHeader />
+    <main class="mall-main">
       <NuxtPage />
     </main>
+    <ShellSiteFooter />
   </div>
 </template>