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/app/shop/not-found.tsx

@@ -0,0 +1,25 @@
+import Link from "next/link";
+
+/**
+ * Shop-specific 404 when an invalid category or sub-category path is requested
+ * (e.g. unknown [category] or [subCategory] in /shop/[category]/[subCategory]).
+ */
+export default function ShopNotFound() {
+  return (
+    <div className="mx-auto max-w-2xl px-4 py-16 text-center">
+      <h1 className="font-[family-name:var(--font-fraunces)] text-2xl font-bold text-[var(--foreground)] md:text-3xl">
+        Page not found
+      </h1>
+      <p className="mt-3 text-[var(--muted)]">
+        This shop category or page doesn&apos;t exist. Try browsing from the
+        shop home.
+      </p>
+      <Link
+        href="/shop"
+        className="mt-6 inline-block rounded-lg bg-[var(--brand-dark)] px-6 py-3 text-sm font-medium text-white transition-colors hover:bg-[var(--brand-hover)]"
+      >
+        Back to Shop
+      </Link>
+    </div>
+  );
+}