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/globals.css

@@ -36,6 +36,8 @@
   --color-checkout-active: #f6a701;
   --color-checkout-done: #1a1a18;
   --color-toc-active-border: #f6a701;
+  --color-success: #2f8f4e;
+  --color-success-subtle: #e8f4ea;
 
   /* Radius */
   --radius-xs: 0.25rem;
@@ -58,7 +60,9 @@
      token (Why-point 4). Tablet floor via the styleguide's 80-85% ratio,
      rounded to 40px. "Product/tool page hero H1" role, distinct from
      --text-h1 (already spoken for: "Seiten-H1 (Blog-Detail, Warenkorb,
-     etc.)" per styleguide.md §2.2 — a different, smaller role). */
+     etc.)" per styleguide.md §2.2 — a different, smaller role). Also used
+     by page-weekly-impulses's hero heading — same 48px role, reused
+     rather than minting a near-duplicate token. */
   --text-h-page: clamp(2.5rem, 1.9286rem + 1.1905vw, 3rem);
   --text-h-page--line-height: 1.15;
   --text-h1: clamp(1.75rem, 1.1786rem + 1.1905vw, 2.25rem);