perf(images): convert remaining <img> tags to next/image project-wide

Clears every remaining @next/next/no-img-element warning — automatic
responsive srcset, lazy-loading, and format optimization instead of
always loading the original file at full size. Fixed-size icons got
explicit width/height; dynamic-aspect photos got fill inside a
relative wrapper.

app/components/AddToCartInlineButton.tsx

@@ -1,6 +1,7 @@
 "use client";
 
 import { useEffect, useRef, useState } from "react";
+import Image from "next/image";
 import { addToCart } from "../lib/cart";
 import { useCartFly } from "./CartFly";
 
@@ -61,7 +62,7 @@ export function AddToCartInlineButton({
       >
         {added ? "Hinzugefügt ✓" : label}
       </span>
-      <img alt="" src="/icon-cart-outline.png" className="h-[1.875rem] w-8 object-contain" />
+      <Image alt="" src="/icon-cart-outline.png" width={32} height={30} className="h-[1.875rem] w-8 object-contain" />
     </button>
   );
 }