perf(booth): optimize assets and result flow

WebP reduces UI payload; JPEG keeps portrait saves compatible.\nFinish ends the visit without prompting another portrait.

components/SpookyAtmosphere.tsx

@@ -18,7 +18,7 @@ export const SpookyAtmosphere: React.FC<SpookyAtmosphereProps> = ({
       {/* 1. Shanghai Dojo Courtyard Night Background Artwork */}
       {/* eslint-disable-next-line @next/next/no-img-element */}
       <img
-        src="/assets/theme/shanghai_dojo_night_bg.jpg"
+        src="/assets/theme/shanghai_dojo_night_bg.webp"
         alt="Shanghai Dojo Night Background"
         className="absolute inset-0 w-full h-full object-cover scale-105"
       />