Move product catalog to Payload CMS

Products now come from Payload's new "products" collection instead of a
hardcoded catalog, same pattern already used for blog posts:

- lib/payload.ts: getProducts()/getProductBySlug() (server-side fetch,
  60s ISR)
- New /api/products route so client components (CartContent,
  RelatedProducts) can reach the same data without a server-only import
- lib/products.ts: useProducts() hook replacing the old PRODUCTS record
- ProductGrid (/shop) fetches server-side directly; now shows all
  catalog products except notizbuch-klarheit (matches Figma's 4-card
  page-shop-overview — still cross-sold via RelatedProducts)
- ProductSpotlight and /todo-cards' Pricing now pull price/photo from
  the same CMS product instead of a separately hardcoded "12,90 €", so
  the two can't silently drift apart
- formatPrice moved to a new lib/format.ts (plain, no "use client") —
  Server Components can't call functions exported from a "use client"
  module directly, which lib/products.ts now is because of the hook

Also fixes two unrelated bugs surfaced along the way: the add-to-cart
button visibly resizing when its "Hinzugefügt ✓" success state showed
(fixed with a CSS-grid text stack sized to the wider of the two
strings), and removes the now-unused local product images from public/.

app/components/AddToCartButton.tsx

@@ -54,7 +54,22 @@ export function AddToCartButton({
       onClick={handleClick}
       className={`${base} ${stateClasses}`}
     >
-      {added ? "Hinzugefügt ✓" : label}
+      {/* CSS-grid text-stack, not just swapping the button's text node
+          directly — this button is inline-flex/content-sized (no w-full),
+          so "Hinzugefügt ✓" being shorter than most labels made the whole
+          button visibly shrink while showing the success state. Stacking
+          both possible texts in the same grid cell (both invisible ones
+          still contribute to sizing) reserves width for whichever is
+          wider, so the button's box never changes size either way. */}
+      <span className="relative grid">
+        <span className="invisible [grid-area:1/1]" aria-hidden="true">
+          {label}
+        </span>
+        <span className="invisible [grid-area:1/1]" aria-hidden="true">
+          Hinzugefügt ✓
+        </span>
+        <span className="[grid-area:1/1]">{added ? "Hinzugefügt ✓" : label}</span>
+      </span>
     </button>
   );
 }