Implement checkout and payment flow with new components

- Added Checkout and Payment pages to handle user authentication and order processing.
- Introduced MockPayPalButton for simulating payment during development.
- Updated CartSheet and CartSidebar components to navigate to the new checkout page.
- Enhanced Cart UI with responsive design for mobile and desktop views.
- Implemented order confirmation and success pages to provide feedback after payment completion.

These changes complete the checkout and payment functionality, improving the overall user experience and ensuring a seamless transition from cart to order confirmation.

app/components/Cart/CartSheet.vue

@@ -27,7 +27,7 @@ async function handleRemoveItem(itemId: string) {
 // Navigate to checkout
 function handleCheckout() {
   close()
-  navigateTo('/kasse')
+  navigateTo('/checkout')
 }
 </script>