Add blog overview page (/blog), link Navbar "Blog" to it

- app/blog/page.tsx: header (title/subheading + bleeding photo with a
  left-edge fade, per Figma node 4577:330), featured-post card (most
  recent), remaining posts as a divided list with category/readTime/date,
  and the reusable Newsletter signup panel. Uses all real posts from
  Payload — the Figma mockup padded its list out to 4 items with two
  posts that were never actually seeded, so this only renders what
  exists (currently 1 featured + 2 listed).
- lib/payload.ts: BlogPost now carries publishedAt (needed for the list's
  per-post date), removed the now-redundant duplicate field on PostDetail.
- Navbar: "Blog" now links to /blog instead of scrolling to the
  homepage's #blog anchor — same real-route pattern as "Shop".
- blog/[slug]/page.tsx: earlier title-size tuning (fluid clamp between
  text-h-feature and text-display) from this session's follow-up
  feedback, not yet pushed.

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

app/components/Navbar.tsx

@@ -33,7 +33,7 @@ function smoothScrollTo(targetY: number) {
 
 const navLinks = [
   { label: "Werkzeuge", href: "#werkzeuge" },
-  { label: "Blog", href: "#blog" },
+  { label: "Blog", href: "/blog" },
   { label: "Über Björn", href: "#ueber-bjoern" },
   { label: "Shop", href: "/shop" },
 ];