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/lib/shipping.ts

@@ -0,0 +1,17 @@
+// Single source of truth for shipping numbers/timeframes — consumed by
+// both the cart's order summary (CartContent.tsx) and the /versand policy
+// page, so the two can never drift apart.
+export const SHIPPING_COST = 2.9;
+export const FREE_SHIPPING_THRESHOLD = 39;
+
+// Kept as an explicit range (not "so schnell wie möglich") per Art. 246a
+// § 1 Abs. 1 Nr. 8 EGBGB — German law requires disclosing a concrete
+// delivery timeframe before contract conclusion, and split into
+// Bearbeitungszeit/Versanddauer so it's clear whether processing time is
+// included in the stated number, not just a single ambiguous figure.
+export const HANDLING_DAYS = { min: 1, max: 2 };
+export const TRANSIT_DAYS_DE = { min: 2, max: 4 };
+export const TOTAL_DAYS_DE = {
+  min: HANDLING_DAYS.min + TRANSIT_DAYS_DE.min,
+  max: HANDLING_DAYS.max + TRANSIT_DAYS_DE.max,
+};