Revert to guest-cart architecture, drop in-site auth/checkout

Per api.meshkee.com/docs/website/AI_PROMPT.md's hard rules (which an
earlier pass on this branch missed): login, register, OTP, the real
cart, addresses, and payment are the customer dashboard's job
(customer.sanihome.ir), not this storefront's. This site should only
ever run a local guest mini-cart and hand off to the dashboard.

- Remove: AuthContext, LoginModal, lib/auth.ts, lib/cart.ts (the real
  /businesses/{id}/cart integration), and the /checkout page entirely.
- Add lib/guestCart.ts: localStorage + shared cookie
  (meshkee-guest-cart, Domain=.sanihome.ir) cart, storeItemVariantId
  keyed, matching the spec's line shape and ~3500-char cookie-size
  cutoff.
- CartContext is now a thin React wrapper over the guest cart (no
  server round-trip, so add/update/remove are synchronous); CartPopup's
  "Continue" builds the exact customer.sanihome.ir/checkout/cart
  (or /login?redirect=...) URL from the spec, base64url-encoding the
  cart and detecting an existing dashboard session via the shared
  meshkee_customer_access_token cookie.
- ProductCard and ProductPurchasePanel add to the guest cart directly;
  no login gate on add-to-cart anymore.
- Comments panel is a plain guest name/email/text form again (the
  comments API never required auth — gating it on the now-removed
  login system was wrong regardless).
- CONTEXT.md rewritten to document the guest-cart contract explicitly
  and flag this exact mistake so it doesn't get reintroduced.

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

src/components/cart/CartButton.tsx

@@ -1,17 +1,14 @@
 "use client";
 
-import { useAuth } from "@/context/AuthContext";
 import { useCart } from "@/context/CartContext";
 
 export function CartButton() {
-  const { user } = useAuth();
   const { itemCount, openPopup } = useCart();
-  const { openLoginModal } = useAuth();
 
   return (
     <button
       type="button"
-      onClick={user ? openPopup : openLoginModal}
+      onClick={openPopup}
       className="relative flex items-center gap-1.5 rounded-lg border border-border px-3 py-2 transition-colors hover:border-red hover:text-red"
     >
       <span aria-hidden>🛒</span>