Fix out-of-stock CTA regressions: grid alignment, preview accuracy, label length

- Replace the invisible reserved-space trick with items-start on every
  product grid (ProductGrid/MerklisteGrid/RelatedProducts) — the
  reservation looked worse in practice (visible dead space under
  in-stock cards' buttons) than letting an out-of-stock card simply be
  taller than its siblings.
- New renderBackInStockHtml() in lib/emailTemplates.ts, used by the
  Live Preview instead of the generic order-status renderer — that one
  showed a fake order number and "Bestellung ansehen", neither of
  which apply to a back-in-stock mail (no order exists). CTA is now
  "Zum Produkt", matching the real backend send.
- Shortened NotifyMeForm's button label ("Benachrichtigen") — the
  longer version wrapped to two lines on narrow single-column cards.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/cart/components/RelatedProducts.tsx

@@ -132,7 +132,7 @@ export function RelatedProducts({
           (opacity: 0) — invisible. Not worth chasing a fix for a
           scroll-reveal nicety on a list that mutates; a static grid
           renders correctly with no animation risk. */}
-      <div className="grid grid-cols-1 sm:grid-cols-12 gap-6 sm:gap-[var(--layout-grid-gap)] w-full max-w-[75rem]">
+      <div className="grid items-start grid-cols-1 sm:grid-cols-12 gap-6 sm:gap-[var(--layout-grid-gap)] w-full max-w-[75rem]">
         {displayProducts.map((product, i) => {
           const discount = discountPercent(product.price, product.compareAtPrice);
           const taxRate = effectiveTaxRate(product, defaultTaxRate);