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/Dockerfile

@@ -0,0 +1,16 @@
+# ── Polaris frontend ─────────────────────────────────────────────
+# Stage 1: build the Vite bundle
+FROM node:20-alpine AS build
+WORKDIR /app
+COPY package.json package-lock.json* ./
+RUN npm install --no-audit --no-fund
+COPY . .
+RUN npm run build
+
+# Stage 2: serve static + proxy /api -> api:8000
+FROM nginx:1.27-alpine
+COPY nginx.conf /etc/nginx/conf.d/default.conf
+COPY --from=build /app/dist /usr/share/nginx/html
+EXPOSE 80
+HEALTHCHECK --interval=10s --timeout=3s --retries=5 CMD wget -qO- http://127.0.0.1/healthz || exit 1
+CMD ["nginx", "-g", "daemon off;"]