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/not-found.tsx

@@ -1,5 +1,6 @@
 import type { Metadata } from "next";
 import Link from "next/link";
+import Image from "next/image";
 import { Reveal, RevealGroup, RevealItem } from "./components/Reveal";
 import { Footer } from "./components/Footer";
 import { TrustRow } from "./components/TrustRow";
@@ -116,9 +117,12 @@ export default function NotFound() {
               actual photographed/rendered note rather than recreating its
               handwriting in CSS. */}
           <Reveal delay={0.15} className="w-full lg:flex-1 flex items-center justify-center">
-            <img
+            <Image
               alt="Sticky Note: 404. Nicht hier. Aber dein Ziel ist sicher nicht weit."
               src="/404-sticky-note.png"
+              width={600}
+              height={689}
+              sizes="(min-width: 1024px) 22rem, 100vw"
               className="w-full max-w-[22rem] rotate-2 hover:rotate-0 transition-transform duration-300"
             />
           </Reveal>
@@ -178,9 +182,12 @@ export default function NotFound() {
             the flat mockup image instead of in Figma. */}
         <Reveal className="relative w-full flex items-stretch h-[14rem] md:h-[16rem] bg-bg-muted overflow-hidden">
           <div className="relative w-full md:w-[45%] shrink-0">
-            <img
+            <Image
               alt=""
               src="/404-testimonial-photo.jpg"
+              width={810}
+              height={291}
+              sizes="(min-width: 768px) 45vw, 100vw"
               className="absolute -inset-1 w-[calc(100%+0.5rem)] h-[calc(100%+0.5rem)] object-cover"
             />
             <div className="hidden md:block absolute inset-y-0 right-0 w-40 bg-gradient-to-l from-bg-muted to-transparent" />