Add blog detail page (/blog/[slug])

- app/blog/[slug]/page.tsx: article header (category/readTime, Playfair
  display title, excerpt, byline), full-bleed hero image, RichText body,
  "Passend dazu: ToDo-Karten" cross-promo, author bio card, "Weiterlesen"
  related-post card. Widths, fonts, and the Passend-dazu card's
  border/padding/spacing were corrected against the actual built Figma
  frame (page-blog-detail, node 4667:344, jCCZyh1DGwdjpv1wGge9To) via
  get_design_context after a few visually-wrong guesses.
- RichText.tsx: new "quote" case renders Lexical's blockquote feature as
  the "Merke dir:" pull-quote — label+underline+divider+Caveat lines,
  using the real exported sparkle/underline assets, matching Figma's
  actual (background-less) structure instead of a guessed bordered card.
- lib/payload.ts: getPostBySlug() for single-post fetches.
- lib/format.ts: formatDate() — "03. Juli 2025"-style German dates.

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

app/lib/payload.ts

@@ -56,6 +56,47 @@ export async function getBlogPosts(limit = 3): Promise<BlogPost[]> {
   }));
 }
 
+export type PostDetail = BlogPost & {
+  content: unknown;
+  publishedAt: string;
+};
+
+type PayloadPostDetail = PayloadPost & { content: unknown; publishedAt: string };
+
+export async function getPostBySlug(slug: string): Promise<PostDetail | null> {
+  const params = new URLSearchParams({
+    "where[tenant.slug][equals]": TENANT_SLUG,
+    "where[slug][equals]": slug,
+    depth: "2",
+    limit: "1",
+  });
+
+  const res = await fetch(`${PAYLOAD_URL}/api/posts?${params}`, {
+    next: { revalidate: 60 },
+  });
+  if (!res.ok) {
+    console.error(`getPostBySlug: Payload returned ${res.status} ${res.statusText}`);
+    return null;
+  }
+
+  const data: { docs?: PayloadPostDetail[] } = await res.json();
+  const doc = data.docs?.[0];
+  if (!doc) return null;
+  return {
+    id: doc.id,
+    title: doc.title,
+    slug: doc.slug,
+    category:
+      typeof doc.category === "object" && doc.category ? doc.category.name : "",
+    readTime: doc.readTime,
+    excerpt: doc.excerpt,
+    thumbnail:
+      typeof doc.thumbnail === "object" && doc.thumbnail ? doc.thumbnail.url : null,
+    content: doc.content,
+    publishedAt: doc.publishedAt,
+  };
+}
+
 // Frontend-facing shape — `id` is Payload's `slug` field, not its numeric
 // row id. Cart items are stored in localStorage keyed by this string (see
 // lib/cart.ts), so slugs were chosen in the Products collection to match