Follow Products.description consolidation to richText-only

Backend dropped the plain-text description field in favor of a single
richText one (see docker/payload commit 1ba8d07). Renders formatted
(bold/italic/multiple paragraphs) via the shared RichText component on
the PDP (der-eine, tasse-die-pause); everywhere else (Passend-dazu cards,
Product JSON-LD, homepage spotlight fallback) derives plain text at read
time via a new extractPlainText() helper instead of a second field.
Removes the description line from cart line items entirely — it only
bloated the cart with no real benefit there.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01J1Hu5bZ1kZUgKhab6yNwCt

app/der-eine/page.tsx

@@ -18,7 +18,7 @@ const title = "Der Eine. – Für die eine Sache, die gerade zählt.";
 // the cart/checkout/JSON-LD elsewhere) — same field, one source now.
 export async function generateMetadata(): Promise<Metadata> {
   const product = await getProductBySlug("stift-kugelschreiber");
-  const description = product?.description ?? undefined;
+  const description = product?.descriptionText || undefined;
   return {
     title,
     description,