Enhance checkout flow with new components and validation

- Added AddressForm and CheckoutForm components for user input during checkout.
- Implemented validation using Zod and VeeValidate for billing address fields.
- Created OrderSummary and MockPayPalButton components for order confirmation and payment simulation.
- Updated CartSheet and CartSidebar to navigate to the new checkout page at '/kasse'.
- Introduced new API endpoints for validating checkout data and creating orders.
- Enhanced user experience with responsive design and error handling.

These changes complete the checkout functionality, allowing users to enter billing information, simulate payment, and confirm orders.

app/middleware/auth.ts

@@ -0,0 +1,28 @@
+// middleware/auth.ts
+
+/**
+ * Authentication middleware
+ *
+ * Protects routes from unauthenticated access
+ *
+ * Usage in pages:
+ *
+ * definePageMeta({
+ *   middleware: 'auth'
+ * })
+ */
+
+export default defineNuxtRouteMiddleware(async (to, from) => {
+  const { loggedIn } = useUserSession()
+
+  // Not logged in - redirect to auth page
+  if (!loggedIn.value) {
+    // Store intended destination for post-login redirect
+    useCookie('redirect_after_login', {
+      maxAge: 600, // 10 minutes
+      path: '/',
+    }).value = to.fullPath
+
+    return navigateTo('/auth')
+  }
+})