Rewrite cart as client-only guest cart per AI_PROMPT.md spec

Replaces the previous server-cart implementation (which wrongly called
the customer-dashboard-only /businesses/{id}/cart* API) with the
documented guest-cart architecture:

- src/lib/guestCart.ts: storage key/cookie name, cookie attributes,
  line shape, and encodeGuestCart() copied verbatim from AI_PROMPT.md
- CartProvider/CartButton: pure client state (localStorage + mirrored
  cookie), no bearer auth, no businessId
- CartButton's "ادامه" builds the exact documented Continue URL,
  branching on hasCustomerSessionCookie() to either
  customer.<domain>/checkout/cart?guestCart=<encoded> or
  .../login?redirect=<encoded checkout path>
- Auth cookie corrected to the real documented name
  (meshkee_customer_access_token, possibly chunked) via a shared
  reassembleChunkedCookie() helper used by both auth.ts (server) and
  authUser.ts (client)
- ProductBuyPanel/product pages switched to the documented
  GET .../store-items/by-product/{id} endpoint (getStoreItemByProduct)
- isInStock() treats stockQuantity === null as unlimited stock, not
  out-of-stock, per spec
- AGENTS.md updated to describe this architecture and remove the
  incorrect server-cart documentation

Manually verified in-browser: badge count, popup lines/qty/remove/
total all update correctly against injected guest-cart data; tsc,
eslint, and next build all pass clean.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/lib/cards.ts

@@ -1,4 +1,4 @@
-import { discountPercent } from "./format";
+import { discountPercent, isInStock } from "./format";
 import { productHref } from "./slug";
 import { DEMO_PRICING, demoPrice } from "./demoPricing";
 import type { ApiProduct, ProductCardData, StoreItem, StoreSpecial } from "./types";
@@ -65,7 +65,7 @@ export function specialToCards(special: StoreSpecial): ProductCardData[] {
         price,
         oldPrice,
         percent: price !== null && oldPrice !== null ? discountPercent(price, oldPrice) : 0,
-        inStock: (variant?.stockQuantity ?? 0) > 0,
+        inStock: isInStock(variant?.stockQuantity),
       };
     });
 }