feat(product-center): implement full product center with NestJS backend, Vue3 admin, and Nuxt4 website page

- NestJS backend: Prisma + PostgreSQL, JWT auth, CRUD for goods/categories/countries/tags/positions, SDS sync with cron, public API, Swagger docs
- Vue3 Admin: Element Plus, goods/categories/countries/tags/positions/sync management pages, batch operations, login with JWT
- Nuxt4 Website: product-center page with sidebar navigation, country filter, search, product grid, pagination, skeleton loading
- Nitro proxy routes for backend API
- 54 backend tests passing
- Updated docs and README

inkreach-official-admin/src/App.vue

@@ -0,0 +1,14 @@
+<script setup lang="ts">
+// Root component renders router-managed views.
+</script>
+
+<template>
+  <router-view />
+</template>
+
+<style>
+#app {
+  width: 100%;
+  height: 100%;
+}
+</style>