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/widerruf/page.tsx

@@ -1,5 +1,6 @@
 import type { Metadata } from "next";
 import Link from "next/link";
+import Image from "next/image";
 import { Reveal } from "../components/Reveal";
 import { Footer } from "../components/Footer";
 import { TrustRow } from "../components/TrustRow";
@@ -42,7 +43,7 @@ export default async function WiderrufPage() {
             {/* Static, not part of the CMS content — same reasoning as
                 the Impressum/Datenschutz/AGB pages' own callout cards. */}
             <div className="bg-bg-muted flex flex-col gap-3 items-start p-6 rounded-md w-full">
-              <img alt="" src="/icon-trust-leaf.png" className="size-7 object-contain" />
+              <Image alt="" src="/icon-trust-leaf.png" width={28} height={28} className="size-7 object-contain" />
               <p
                 className="font-semibold text-body text-text-primary"
                 style={{ fontFamily: "var(--font-lora)" }}