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/hooks/useStoreUserEffect.ts

@@ -0,0 +1,31 @@
+"use client";
+
+import { useUser } from "@clerk/nextjs";
+import { useConvexAuth, useMutation } from "convex/react";
+import { useEffect, useState } from "react";
+import { api } from "../../../../convex/_generated/api";
+import { Id } from "../../../../convex/_generated/dataModel";
+
+export function useStoreUserEffect() {
+  const { isLoading, isAuthenticated } = useConvexAuth();
+  const { user } = useUser();
+  const [userId, setUserId] = useState<Id<"users"> | null>(null);
+  const storeUser = useMutation(api.users.store);
+
+  useEffect(() => {
+    if (!isAuthenticated) return;
+    let cancelled = false;
+    storeUser().then((id) => {
+      if (!cancelled) setUserId(id);
+    });
+    return () => {
+      cancelled = true;
+      setUserId(null);
+    };
+  }, [isAuthenticated, storeUser, user?.id]);
+
+  return {
+    isLoading: isLoading || (isAuthenticated && userId === null),
+    isAuthenticated: isAuthenticated && userId !== null,
+  };
+}