Add guest checkout and admin login link

- Add guest checkout flow: customers can now complete orders without creating an account
  - New /checkout page with login or guest options
  - Guest form collects: full name, email, phone, address
  - Orders track guest info via guestName and guestPhone fields
- Update cart message to reflect guest option
- Add "Admin login" link to customer login page for clear navigation
- Update checkout API to accept and store guest information

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/app/cart/page.tsx

@@ -7,8 +7,6 @@ import Price from '@/components/Price';
 
 export default function CartPage() {
   const [mounted, setMounted] = useState(false);
-  const [checkingOut, setCheckingOut] = useState(false);
-  const [checkoutError, setCheckoutError] = useState<string | null>(null);
   const items = useCart((s) => s.items);
   const removeItem = useCart((s) => s.removeItem);
   const setQuantity = useCart((s) => s.setQuantity);
@@ -17,24 +15,8 @@ export default function CartPage() {
 
   useEffect(() => setMounted(true), []);
 
-  const handleCheckout = async () => {
-    setCheckingOut(true);
-    setCheckoutError(null);
-    try {
-      const res = await fetch('/api/checkout', {
-        method: 'POST',
-        headers: { 'Content-Type': 'application/json' },
-        body: JSON.stringify({ items }),
-      });
-      const data = await res.json();
-      if (!res.ok || !data.url) {
-        throw new Error(data.error ?? 'Something went wrong starting checkout.');
-      }
-      window.location.href = data.url;
-    } catch (err) {
-      setCheckoutError(err instanceof Error ? err.message : 'Something went wrong.');
-      setCheckingOut(false);
-    }
+  const handleCheckout = () => {
+    window.location.href = '/checkout';
   };
 
   if (!mounted) return null; // avoid a hydration mismatch on first paint
@@ -180,16 +162,14 @@ export default function CartPage() {
 
         <div className="w-full max-w-xs">
           <div className="mb-4 rounded border border-splash-blue/40 bg-splash-blue/5 px-3 py-2 text-xs text-muted">
-            Your cart will be saved. You&apos;ll need to log in again to complete your order.
+            Your cart will be saved. You can log in or continue as a guest to complete your order.
           </div>
           <button
             onClick={handleCheckout}
-            disabled={checkingOut}
-            className="w-full bg-clay px-6 py-3 text-sm font-medium text-paper transition-colors hover:bg-clay-dark disabled:opacity-60"
+            className="w-full bg-clay px-6 py-3 text-sm font-medium text-paper transition-colors hover:bg-clay-dark"
           >
-            {checkingOut ? 'Redirecting to checkout…' : 'Checkout'}
+            Checkout
           </button>
-          {checkoutError && <p className="mt-2 text-center text-xs text-splash-pink">{checkoutError}</p>}
           <p className="mt-2 text-center text-xs text-muted">You&apos;ll enter payment details on the next step.</p>
           <p className="mt-1 text-center text-xs text-muted">
             By placing an order you agree to our{' '}