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/format.ts

@@ -15,6 +15,13 @@ export function discountPercent(price: number, oldPrice: number): number {
   return Math.round(((oldPrice - price) / oldPrice) * 100);
 }
 
+// Per AI_PROMPT.md "Shopping cart" step 3: a variant's stockQuantity of
+// `null` means unlimited stock (in stock), not zero/out of stock — never
+// treat it as falsy.
+export function isInStock(stockQuantity: number | null | undefined): boolean {
+  return stockQuantity === null || (stockQuantity ?? 0) > 0;
+}
+
 // The static-image-slot API documents aspectRatio as "16:9", but CSS's
 // aspect-ratio property needs a slash ("16/9"), not a colon.
 export function toCssAspectRatio(value: string | number | null | undefined): string | undefined {