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/components/ui/Tabs.vue

@@ -0,0 +1,45 @@
+<script setup lang="ts">
+defineProps<{ tabs: { key: string; label: string }[]; modelValue: string }>();
+const emit = defineEmits<{ "update:modelValue": [key: string] }>();
+</script>
+
+<template>
+  <div class="tabs">
+    <div class="tab-head">
+      <button
+        v-for="tab in tabs"
+        :key="tab.key"
+        type="button"
+        :class="{ on: tab.key === modelValue }"
+        @click="emit('update:modelValue', tab.key)"
+      >{{ tab.label }}</button>
+    </div>
+    <div class="tab-body">
+      <slot :active="modelValue" />
+    </div>
+  </div>
+</template>
+
+<style scoped>
+.tab-head {
+  display: flex;
+  border-bottom: 1px solid var(--mall-line);
+}
+.tab-head button {
+  border: 0;
+  background: none;
+  padding: 12px 24px;
+  font-size: 14px;
+  cursor: pointer;
+  color: var(--mall-muted);
+  border-bottom: 2px solid transparent;
+}
+.tab-head button.on {
+  color: var(--mall-red);
+  border-bottom-color: var(--mall-red);
+  font-weight: 600;
+}
+.tab-body {
+  padding: 20px 0;
+}
+</style>