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/cart.ts

@@ -1,11 +1,12 @@
 "use client";
 
-import { useEffect, useState } from "react";
+import { useSyncExternalStore } from "react";
 
 const CART_KEY = "ep_cart";
 const CART_EVENT = "ep-cart-updated";
+const EMPTY_CART: CartItem[] = [];
 
-type CartItem = { id: string; qty: number };
+export type CartItem = { id: string; qty: number };
 
 function readCart(): CartItem[] {
   if (typeof window === "undefined") return [];
@@ -34,23 +35,66 @@ export function addToCart(id: string, qty = 1) {
   writeCart(items);
 }
 
-// Reactive cart item count — 0 until the client hydrates and reads
-// localStorage, then stays in sync across tabs ("storage") and same-tab
-// updates (the CART_EVENT dispatched by addToCart, which "storage" alone
-// doesn't fire for the tab that made the change).
-export function useCartCount(): number {
-  const [count, setCount] = useState(0);
-
-  useEffect(() => {
-    setCount(getCartCount());
-    const onUpdate = () => setCount(getCartCount());
-    window.addEventListener(CART_EVENT, onUpdate);
-    window.addEventListener("storage", onUpdate);
-    return () => {
-      window.removeEventListener(CART_EVENT, onUpdate);
-      window.removeEventListener("storage", onUpdate);
-    };
-  }, []);
-
-  return count;
+export function removeFromCart(id: string) {
+  writeCart(readCart().filter((i) => i.id !== id));
+}
+
+// qty <= 0 removes the item outright — the cart page's quantity stepper
+// never lets the visible count go below 1, but this keeps the function
+// itself safe to call with any integer without a separate remove path.
+export function setQuantity(id: string, qty: number) {
+  if (qty <= 0) {
+    removeFromCart(id);
+    return;
+  }
+  const items = readCart();
+  const existing = items.find((i) => i.id === id);
+  if (existing) existing.qty = qty;
+  writeCart(items);
+}
+
+// Cached-by-raw-string snapshot, not a fresh JSON.parse() every call —
+// useSyncExternalStore (below) requires getSnapshot to return the SAME
+// reference when the underlying data hasn't actually changed, or React
+// treats every render as a change. readCart()'s plain JSON.parse would
+// allocate a new array each call and break that.
+let cachedRaw: string | null | undefined;
+let cachedItems: CartItem[] = EMPTY_CART;
+
+export function getCart(): CartItem[] {
+  if (typeof window === "undefined") return EMPTY_CART;
+  const raw = window.localStorage.getItem(CART_KEY);
+  if (raw === cachedRaw) return cachedItems;
+  cachedRaw = raw;
+  try {
+    cachedItems = raw ? JSON.parse(raw) : EMPTY_CART;
+  } catch {
+    cachedItems = EMPTY_CART;
+  }
+  return cachedItems;
+}
+
+function subscribe(onStoreChange: () => void) {
+  window.addEventListener(CART_EVENT, onStoreChange);
+  window.addEventListener("storage", onStoreChange);
+  return () => {
+    window.removeEventListener(CART_EVENT, onStoreChange);
+    window.removeEventListener("storage", onStoreChange);
+  };
+}
+
+// useSyncExternalStore, not useState+useEffect — localStorage is an
+// external store outside React, and the previous approach (setState
+// synchronously inside an effect body) causes an extra render and trips
+// the react-hooks/set-state-in-effect lint rule. This is React's own
+// recommended pattern for subscribing to exactly this kind of external
+// store, and is correctly SSR-safe via the third (server snapshot)
+// argument — 0 / EMPTY_CART until the client hydrates and reads
+// localStorage for real.
+export function useCartCount(): number {
+  return useSyncExternalStore(subscribe, getCartCount, () => 0);
+}
+
+export function useCart(): CartItem[] {
+  return useSyncExternalStore(subscribe, getCart, () => EMPTY_CART);
 }