catalyst Add support for Stencil-style add-to-cart URLs

What/Why?

Adds support for BigCommerce Stencil-style add-to-cart URLs via Next.js route handlers. This enables adding products to cart using query parameters, matching the behavior of Stencil stores for compatibility.

Changes

New route handlers:
- /cart.php - Stencil compatibility route
- /add-to-cart - Catalyst-native route
Shared handler logic:
- Product lookup by product_id or sku
- Quantity support via qty parameter
- Coupon code application via couponcode parameter
- Action-based redirects (add → cart, buy → checkout)

Features

✅ Product lookup by ID or SKU ✅ Quantity parameter support ✅ Coupon code application ✅ Action-based redirects (cart/checkout) ✅ Comprehensive error handling ✅ User-friendly error messages via server toasts ✅ Optimized SKU search (limited to 10 products, stops at first match) ✅ Verifies checkout exists before applying coupons

Supported URL Patterns

/cart.php?action=add&product_id=123
/cart.php?action=buy&sku=xlredtshirt
/add-to-cart?action=add&product_id=123&qty=3
/add-to-cart?action=add&product_id=123&couponcode=10off100

Implementation Details

Uses Next.js Route Handlers (optimal for GET requests and redirects)
Leverages existing Catalyst utilities (addToOrCreateCart, getCartId, applyCouponCode)
GraphQL queries for product lookup and validation
Error handling follows Catalyst patterns (matches checkout/route.ts)
SKU search uses searchProducts API (searches Name, SKU, and Description)