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/categories.ts

@@ -0,0 +1,11 @@
+import { QueryCtx } from "../_generated/server";
+import { Id } from "../_generated/dataModel";
+
+export async function getCategoryOrThrow(
+  ctx: QueryCtx,
+  id: Id<"categories">,
+) {
+  const category = await ctx.db.get(id);
+  if (!category) throw new Error("Category not found");
+  return category;
+}