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/shell/SiteFooter.vue

@@ -0,0 +1,73 @@
+<script setup lang="ts">
+const { t } = useI18n();
+const visible = ref(false);
+
+function onScroll(): void {
+  visible.value = window.scrollY > 400;
+}
+function toTop(): void {
+  window.scrollTo({ top: 0, behavior: "smooth" });
+}
+onMounted(() => window.addEventListener("scroll", onScroll, { passive: true }));
+onBeforeUnmount(() => window.removeEventListener("scroll", onScroll));
+</script>
+
+<template>
+  <footer class="foot">
+    <div class="w1200">
+      <ul class="props">
+        <li>{{ t("shell.footer.v1") }}</li>
+        <li>{{ t("shell.footer.v2") }}</li>
+        <li>{{ t("shell.footer.v3") }}</li>
+        <li>{{ t("shell.footer.v4") }}</li>
+      </ul>
+      <div class="copyright">
+        <p>{{ t("shell.footer.copyright") }}</p>
+        <p>{{ t("shell.footer.icp") }}</p>
+      </div>
+    </div>
+    <button v-show="visible" class="backtop" type="button" :aria-label="t('shell.backToTop')" @click="toTop">↑</button>
+  </footer>
+</template>
+
+<style scoped>
+.foot {
+  padding-top: 10px;
+  text-align: center;
+}
+.props {
+  margin: 40px auto 0;
+  width: 720px;
+  border-top: 1px solid var(--mall-line);
+  padding: 40px 0 0;
+  list-style: none;
+  display: flex;
+  justify-content: space-between;
+  font-size: 14px;
+}
+.copyright {
+  font-size: 12px;
+  line-height: 28px;
+  color: var(--mall-muted);
+  padding: 30px 0 20px;
+}
+.backtop {
+  position: fixed;
+  right: 40px;
+  bottom: 60px;
+  width: 44px;
+  height: 44px;
+  border: 1px solid var(--mall-line-dark);
+  background: #fff;
+  border-radius: 4px;
+  cursor: pointer;
+  font-size: 18px;
+  color: var(--mall-muted);
+  z-index: 800;
+}
+.backtop:hover {
+  color: #fff;
+  background: var(--mall-red);
+  border-color: var(--mall-red);
+}
+</style>