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/QtyStepper.vue

@@ -0,0 +1,57 @@
+<script setup lang="ts">
+const props = withDefaults(defineProps<{ modelValue: number; max?: number }>(), { max: 999 });
+const emit = defineEmits<{ "update:modelValue": [v: number] }>();
+
+function set(v: number): void {
+  const next = Math.min(props.max, Math.max(1, Math.floor(v) || 1));
+  emit("update:modelValue", next);
+}
+</script>
+
+<template>
+  <div class="stepper">
+    <button type="button" :disabled="modelValue <= 1" @click="set(modelValue - 1)">−</button>
+    <input
+      :value="modelValue"
+      type="number"
+      :min="1"
+      :max="max"
+      @change="set(Number(($event.target as HTMLInputElement).value))"
+    />
+    <button type="button" :disabled="modelValue >= max" @click="set(modelValue + 1)">+</button>
+  </div>
+</template>
+
+<style scoped>
+.stepper {
+  display: inline-flex;
+  border: 1px solid var(--mall-line-dark);
+  border-radius: var(--mall-radius);
+  overflow: hidden;
+}
+button {
+  width: 30px;
+  height: 32px;
+  border: 0;
+  background: #fafafa;
+  cursor: pointer;
+  font-size: 14px;
+}
+button:disabled {
+  opacity: 0.4;
+  cursor: not-allowed;
+}
+input {
+  width: 48px;
+  height: 32px;
+  border: 0;
+  border-left: 1px solid var(--mall-line);
+  border-right: 1px solid var(--mall-line);
+  text-align: center;
+  font-size: 13px;
+  outline: none;
+}
+input::-webkit-inner-spin-button {
+  display: none;
+}
+</style>