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/cart/components/CartContent.tsx

@@ -40,6 +40,10 @@ export function CartContent({
     .filter((row): row is { entry: typeof cart[number]; 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 shipping =
     items.length === 0 || (freeShippingThreshold !== null && subtotal >= freeShippingThreshold)
       ? 0
@@ -187,6 +191,14 @@ export function CartContent({
                 <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 flex-col gap-0.5 w-full">
                 <div className="flex items-center w-full">
                   <span className="flex items-center gap-1.5 text-body-sm text-text-primary">