feat: initial commit — storefront, convex backend, and shared packages

Completes the first milestone of The Pet Loft ecommerce platform:
- apps/storefront: full customer-facing Next.js app with HeroUI (cart,
  checkout, orders, wishlist, product detail, shop, search, auth)
- convex/: serverless backend with schema, queries, mutations, actions,
  HTTP routes, Stripe/Shippo integrations, and co-located tests
- packages/types, packages/utils, packages/convex: shared workspace packages

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/storefront/src/components/layout/header/desktop/DesktopHeader.tsx

@@ -0,0 +1,13 @@
+import { TopUtilityBar } from "./TopUtilityBar";
+import { CoreBrandBar } from "./CoreBrandBar";
+import { BottomNav } from "./BottomNav";
+
+export function DesktopHeader() {
+  return (
+    <header className="sticky top-0 z-50 w-full shadow-sm">
+      <TopUtilityBar />
+      <CoreBrandBar />
+      <BottomNav />
+    </header>
+  );
+}