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.

server/api/checkout/validate.post.ts

@@ -0,0 +1,74 @@
+/**
+ * POST /api/checkout/validate
+ *
+ * Validates checkout data before creating an order
+ *
+ * Request Body:
+ * {
+ *   salutation: 'male' | 'female' | 'other',
+ *   firstName: string,
+ *   lastName: string,
+ *   dateOfBirth: string (YYYY-MM-DD),
+ *   street: string,
+ *   postCode: string (5 digits),
+ *   city: string,
+ *   countryCode: string (ISO 3166-1 alpha-2, default: 'DE'),
+ *   saveAddress: boolean (optional)
+ * }
+ *
+ * Response:
+ * {
+ *   success: true,
+ *   message: string
+ * }
+ *
+ * Errors:
+ * - 401: Not authenticated
+ * - 400: Empty cart
+ * - 422: Validation errors
+ */
+
+import { checkoutSchema } from '../../utils/schemas/checkout'
+
+export default defineEventHandler(async (event) => {
+  // 1. Require authentication
+  const { user } = await requireUserSession(event)
+
+  // 2. Check if cart has items
+  const cart = await getOrCreateCart(event)
+  const cartSummary = await getCartWithItems(cart.id)
+
+  if (cartSummary.itemCount === 0) {
+    throw createError({
+      statusCode: 400,
+      statusMessage: 'Cart is empty',
+      message: 'Dein Warenkorb ist leer. Füge Produkte hinzu, um fortzufahren.',
+    })
+  }
+
+  // 3. Validate checkout data
+  const body = await readBody(event)
+
+  try {
+    const validatedData = await checkoutSchema.parseAsync(body)
+
+    return {
+      success: true,
+      message: 'Checkout-Daten sind gültig',
+      data: validatedData,
+    }
+  } catch (error: any) {
+    // Zod validation errors
+    if (error.errors) {
+      throw createError({
+        statusCode: 422,
+        statusMessage: 'Validation error',
+        message: 'Bitte überprüfe deine Eingaben',
+        data: error.errors,
+      })
+    }
+
+    // Unknown error
+    throw error
+  }
+})