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/components/AddToCartButton.tsx

@@ -0,0 +1,60 @@
+"use client";
+
+import { useEffect, useRef, useState } from "react";
+import { addToCart } from "../lib/cart";
+import { useCartFly } from "./CartFly";
+
+const FEEDBACK_MS = 2000;
+const PRODUCT_ID = "todo-karten";
+
+/**
+ * Shared by /todo-cards's Hero + pricing panel and Home's product
+ * spotlight. Used to navigate straight to /cart on click; now stays on the
+ * page instead (matching AddToCartInlineButton's behavior everywhere else)
+ * — adds to the cart, plays the fly-to-navbar-icon animation, and shows a
+ * brief inline success state instead of leaving the page.
+ */
+export function AddToCartButton({
+  label,
+  className,
+}: {
+  label: string;
+  className?: string;
+}) {
+  const [added, setAdded] = useState(false);
+  const timeoutRef = useRef<ReturnType<typeof setTimeout> | undefined>(undefined);
+  const buttonRef = useRef<HTMLButtonElement>(null);
+  const { fly } = useCartFly();
+
+  useEffect(() => () => clearTimeout(timeoutRef.current), []);
+
+  function handleClick() {
+    addToCart(PRODUCT_ID);
+    if (buttonRef.current) fly(buttonRef.current);
+    setAdded(true);
+    clearTimeout(timeoutRef.current);
+    timeoutRef.current = setTimeout(() => setAdded(false), FEEDBACK_MS);
+  }
+
+  const base =
+    className ??
+    "inline-flex items-center justify-center px-6 py-[0.8125rem] rounded-sm bg-brand hover:bg-brand-hover active:scale-[0.97] transition-all font-bold text-body text-text-primary whitespace-nowrap focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-brand focus-visible:ring-offset-2 focus-visible:ring-offset-bg-base";
+  // Trailing `!` — Tailwind v4's important-modifier syntax moved from a
+  // leading `!` to this trailing suffix — forces the success color to win
+  // over whatever bg-brand/text-text-primary each caller already baked
+  // into `base`, since appending on top can't rely on CSS source order
+  // the way branching a whole className (AddToCartInlineButton's
+  // approach) can when the base itself varies per caller.
+  const stateClasses = added ? "bg-success! hover:bg-success! text-white!" : "";
+
+  return (
+    <button
+      ref={buttonRef}
+      type="button"
+      onClick={handleClick}
+      className={`${base} ${stateClasses}`}
+    >
+      {added ? "Hinzugefügt ✓" : label}
+    </button>
+  );
+}