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/bestellbestaetigung/components/BestellbestaetigungContent.tsx

@@ -270,9 +270,12 @@ export function BestellbestaetigungContent() {
               gap shows the image itself, not whatever's behind it — the
               parent's overflow-hidden clips the small overflow back
               down to a clean box either way. */}
-          <img
+          <Image
             alt=""
             src="/bestellbestaetigung-testimonial-photo.jpg"
+            width={366}
+            height={126}
+            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" />