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/src/components/Spinner.tsx

@@ -0,0 +1,17 @@
+export function Spinner({ className = 'h-5 w-5' }: { className?: string }) {
+  return (
+    <svg className={`animate-spin ${className}`} viewBox="0 0 24 24" fill="none" aria-hidden="true">
+      <circle cx="12" cy="12" r="10" stroke="currentColor" strokeWidth="3" className="opacity-20" />
+      <path d="M22 12a10 10 0 0 1-10 10" stroke="currentColor" strokeWidth="3" strokeLinecap="round" />
+    </svg>
+  );
+}
+
+export function PageLoader({ label = 'Loading…' }: { label?: string }) {
+  return (
+    <div className="flex flex-col items-center justify-center gap-3 py-24 text-slate-400">
+      <Spinner className="h-7 w-7 text-indigo-400" />
+      <p className="text-sm">{label}</p>
+    </div>
+  );
+}