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>

convex/model/stripe.ts

@@ -0,0 +1,24 @@
+"use node";
+
+import Stripe from "stripe";
+
+const stripe = new Stripe(process.env.STRIPE_SECRET_KEY!);
+
+export async function getOrCreateStripeCustomer(input: {
+  stripeCustomerId: string | undefined;
+  email: string;
+  name: string;
+  convexUserId: string;
+}): Promise<string> {
+  if (input.stripeCustomerId) {
+    return input.stripeCustomerId;
+  }
+
+  const customer = await stripe.customers.create({
+    email: input.email,
+    name: input.name,
+    metadata: { convexUserId: input.convexUserId },
+  });
+
+  return customer.id;
+}