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/konto/merkliste/components/MerklisteGrid.tsx

@@ -42,7 +42,7 @@ export function MerklisteGrid({
   }
 
   return (
-    <RevealGroup className="grid grid-cols-1 sm:grid-cols-2 lg:grid-cols-3 gap-6 w-full">
+    <RevealGroup className="grid items-start grid-cols-1 sm:grid-cols-2 lg:grid-cols-3 gap-6 w-full">
       {visibleEntries.map(({ item, product }) => {
         const discount = discountPercent(product.price, product.compareAtPrice);
         const taxRate = effectiveTaxRate(product, defaultTaxRate);