feat(cart, checkout, bestellbestaetigung): show total savings in the order summary

"Du sparst -X,XX €" line under Zwischensumme, summing (compareAtPrice
- price) × qty across items with a discount — only shown when > 0,
same conditional-render convention as the rest of the sale-pricing
UI (badge/strikethrough already gated on discountPercent() !== null).

app/bestellbestaetigung/components/BestellbestaetigungContent.tsx

@@ -5,7 +5,7 @@ import Link from "next/link";
 import Image from "next/image";
 import type { CartItem } from "../../lib/cart";
 import { useProducts } from "../../lib/products";
-import { formatPrice, formatDate } from "../../lib/format";
+import { formatPrice, formatDate, discountPercent } from "../../lib/format";
 import { Reveal } from "../../components/Reveal";
 import { CheckoutSteps } from "../../components/CheckoutSteps";
 import { ORDER_KEY, type OrderSnapshot } from "../../lib/order";
@@ -96,6 +96,10 @@ export function BestellbestaetigungContent() {
     .filter((row): row is { entry: CartItem; product: NonNullable<(typeof row)["product"]> } => Boolean(row.product));
 
   const subtotal = items.reduce((sum, { entry, product }) => sum + entry.qty * product.price, 0);
+  const totalSavings = items.reduce((sum, { entry, product }) => {
+    const discount = discountPercent(product.price, product.compareAtPrice);
+    return discount !== null ? sum + entry.qty * (product.compareAtPrice! - product.price) : sum;
+  }, 0);
   const total = subtotal + order.shippingCost;
 
   return (
@@ -194,6 +198,13 @@ export function BestellbestaetigungContent() {
                 <span className="flex-1" />
                 <span className="text-body-sm text-text-primary">{formatPrice(subtotal)}</span>
               </div>
+              {totalSavings > 0 && (
+                <div className="flex items-center w-full">
+                  <span className="text-body-sm text-success">Du sparst</span>
+                  <span className="flex-1" />
+                  <span className="font-bold text-body-sm text-success">-{formatPrice(totalSavings)}</span>
+                </div>
+              )}
               <div className="flex items-center w-full">
                 <span className="text-body-sm text-text-primary">Versand</span>
                 <span className="flex-1" />