perf(landing): replace low-res hero mockup PNG with a retina WebP

The old public/app-preview-mobile.png was exactly 327x714px -- 1:1 with
its CSS display size, so retina displays (2x/3x DPR) had no headroom and
Lighthouse flagged it as "serves images with low resolution"
(image-size-responsive, SpeedUp-Plan P2).

Replaced with a fresh feed screenshot supplied by the user, center-cropped
to the same 327:714 aspect ratio and delivered at 981x2142 (3x) as WebP
(~193 KB) so next/image's built-in optimizer has real headroom to generate
a proper srcset instead of upscaling a 1x source.

src/app/page.tsx

@@ -90,7 +90,7 @@ export default async function RootPage() {
                     <span className="h-6 w-[88px] rounded-full bg-black" aria-hidden="true" />
                   </div>
                   <Image
-                    src="/app-preview-mobile.png"
+                    src="/app-preview-mobile.webp"
                     alt={t("previewAlt")}
                     width={327}
                     height={714}