Add Polaris frontend: storefront + admin console (React 18/Vite/TS/Tailwind)

- Shared src/lib/api.ts fetch wrapper -> /api with Cache-Control: no-store
- Storefront: home hero, shop grid (category filter + search), product detail,
  cart, checkout (POST /api/orders -> confirmation + order number), order tracking
- Admin: JWT login, today KPI dashboard (revenue/orders/gross/net/margin/aov/refunds),
  top products, supplier performance, product import, lifecycle view, order list
  with tracking simulation
- Multi-stage Dockerfile (Vite build -> nginx static + /api proxy -> api:8000)
- nginx.conf with SPA fallback and no-store HTML caching
- mock/mock_api.py for local verification (excluded from image)

frontend/postcss.config.js

@@ -0,0 +1,6 @@
+export default {
+  plugins: {
+    tailwindcss: {},
+    autoprefixer: {},
+  },
+}