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/components/Header.tsx

@@ -51,10 +51,7 @@ export default function Header({
 
           <div className="mr-auto hidden items-center gap-2 lg:flex">
             <AccountMenu user={user} />
-            <CartButton
-              user={user}
-              className="flex h-11 items-center gap-2 rounded-xl bg-brand px-3 text-[13px] text-white transition hover:bg-brand-dark"
-            />
+            <CartButton className="flex h-11 items-center gap-2 rounded-xl bg-brand px-3 text-[13px] text-white transition hover:bg-brand-dark" />
           </div>
         </div>