feat: add /bestellbestaetigung order confirmation page

Matches the Figma mockup (checkmark hero, order summary card,
delivery-status panel, testimonial band) with the checkout's 4-step
bar inserted (all steps done) and the "Bis dahin: Lass dich
inspirieren" block omitted, per request.

Extracted the step bar into a shared CheckoutSteps component so
/checkout and /bestellbestaetigung don't duplicate it. The actually-
selected shipping cost and payment method are captured into a
sessionStorage snapshot by /checkout's "Jetzt kaufen" click (there's
no real order backend, so this click is what "placing the order"
means here) and read back on the confirmation page — not just
defaulted to the first active method of each, so the receipt matches
what the shopper actually picked. Also tightened the checkout
newsletter-consent copy ("Wenn du zustimmst" instead of "Wenn du
oben zustimmst").

app/lib/cart.ts

@@ -39,6 +39,14 @@ export function removeFromCart(id: string) {
   writeCart(readCart().filter((i) => i.id !== id));
 }
 
+// Called by /bestellbestaetigung once it has captured a snapshot of the
+// cart to display — there's no real order backend here, so "placing an
+// order" just means the local cart empties out the same way it would
+// after a real purchase completes.
+export function clearCart() {
+  writeCart([]);
+}
+
 // qty <= 0 removes the item outright — the cart page's quantity stepper
 // never lets the visible count go below 1, but this keeps the function
 // itself safe to call with any integer without a separate remove path.