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/vite.config.ts

@@ -0,0 +1,22 @@
+import { defineConfig } from 'vite'
+import react from '@vitejs/plugin-react'
+
+// VITE_API_URL defaults to '/api' — nginx proxies /api -> api:8000 in the
+// compose network. For local dev, the proxy below forwards /api to a
+// backend running on localhost:8000.
+export default defineConfig({
+  plugins: [react()],
+  server: {
+    port: 5173,
+    proxy: {
+      '/api': {
+        target: 'http://localhost:8000',
+        changeOrigin: true,
+      },
+    },
+  },
+  build: {
+    outDir: 'dist',
+    sourcemap: false,
+  },
+})