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/nginx.conf

@@ -0,0 +1,42 @@
+server {
+    listen 80;
+    server_name _;
+
+    root /usr/share/nginx/html;
+    index index.html;
+
+    gzip on;
+    gzip_vary on;
+    gzip_min_length 1024;
+    gzip_types text/plain text/css application/javascript application/json image/svg+xml;
+
+    location = /healthz {
+        access_log off;
+        default_type text/plain;
+        return 200 "ok";
+    }
+
+    # API traffic → FastAPI backend (compose service `api` on :8000)
+    location /api/ {
+        proxy_pass http://api:8000;
+        proxy_http_version 1.1;
+        proxy_set_header Host $host;
+        proxy_set_header X-Real-IP $remote_addr;
+        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+        proxy_set_header X-Forwarded-Proto $scheme;
+        proxy_read_timeout 60s;
+    }
+
+    # Hashed build assets: cache aggressively
+    location /assets/ {
+        expires 30d;
+        add_header Cache-Control "public, max-age=2592000, immutable";
+        try_files $uri =404;
+    }
+
+    # SPA fallback; never cache the HTML shell
+    location / {
+        try_files $uri $uri/ /index.html;
+        add_header Cache-Control "no-store";
+    }
+}