Add shop, cart, versand pages and Impulse & Tipps detail page

- New /shop overview, /cart (real cart state via useSyncExternalStore),
  /versand (shipping policy page with TOC) and /newsletter detail page
- Cart: quantity/removal, order summary, related-products cross-sell
  with randomized picks, VersandModal quick-reference instead of
  navigating away, MwSt. disclosure next to unit prices
- Add-to-cart UX: inline success feedback (green state) plus a
  fly-to-navbar-cart-icon animation (CartFlyProvider) with a delayed
  badge count-up; AddToCartButton no longer navigates straight to /cart
- Shared lib/products.ts catalog and lib/shipping.ts constants (cost,
  free-shipping threshold, handling/transit days) so cart, trust badges
  and the versand page can never drift apart
- Fix Navbar smooth-scroll easing (ease-out instead of ease-in-out, no
  more perceived start delay); compress newsletter modal photo
  2.4MB -> 85KB to fix first-open jank

app/page.tsx

@@ -2,6 +2,7 @@ import type { Metadata } from "next";
 import { Hero } from "./components/Hero";
 import { Divider } from "./components/Divider";
 import { Tools } from "./components/Tools";
+import { ProductSpotlight } from "./components/ProductSpotlight";
 import { Blog } from "./components/Blog";
 import { About } from "./components/About";
 import { Newsletter } from "./components/Newsletter";
@@ -37,6 +38,7 @@ export default function Home() {
         <Hero />
         <Divider />
         <Tools />
+        <ProductSpotlight />
         <Blog />
         <About />
         <Newsletter />