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/index.html

@@ -0,0 +1,21 @@
+<!doctype html>
+<html lang="en" class="dark">
+  <head>
+    <meta charset="UTF-8" />
+    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+    <meta
+      name="description"
+      content="Polaris — automated e-commerce. Curated products, transparent pricing, and live order tracking from supplier to doorstep."
+    />
+    <link
+      rel="icon"
+      type="image/svg+xml"
+      href="data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 24 24'%3E%3Cpath fill='%23818cf8' d='M12 2.2l2.95 6.3 6.85.87-5.04 4.73 1.29 6.78L12 17.6l-6.05 3.28 1.29-6.78L2.2 9.37l6.85-.87z'/%3E%3C/svg%3E"
+    />
+    <title>Polaris — Automated Commerce</title>
+  </head>
+  <body>
+    <div id="root"></div>
+    <script type="module" src="/src/main.tsx"></script>
+  </body>
+</html>