Fix checkout loop issue caused by type mismatch in mode state

Critical fix: Update mode state type to include 'logged-in'.

The mode state was initialized with 'logged-in' but the TypeScript type
only allowed 'login' | 'guest' | null, causing the checkout flow to behave
unexpectedly and loop/clear the cart.

Type changed from:
  useState<'login' | 'guest' | null>(...)
To:
  useState<'logged-in' | 'login' | 'guest' | null>(...)

This fixes the checkout issue where customers couldn't complete purchases.

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

src/app/checkout/client.tsx

@@ -13,7 +13,7 @@ interface CheckoutPageClientProps {
 
 export default function CheckoutPageClient({ isLoggedIn, customer }: CheckoutPageClientProps) {
   const [mounted, setMounted] = useState(false);
-  const [mode, setMode] = useState<'login' | 'guest' | null>(isLoggedIn ? 'logged-in' : null);
+  const [mode, setMode] = useState<'logged-in' | 'login' | 'guest' | null>(isLoggedIn ? 'logged-in' : null);
   const [checkingOut, setCheckingOut] = useState(false);
   const [checkoutError, setCheckoutError] = useState<string | null>(null);
   const [guestData, setGuestData] = useState({