perf(booth): optimize assets and result flow

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

lib/watermark.ts

@@ -16,7 +16,7 @@ export async function applyWatermarkToFile(filePath: string): Promise<void> {
 
   try {
     const baseDir = process.cwd();
-    const logoPath = path.join(baseDir, "public", "assets", "brand", "shanghai_karate_logo.png");
+    const logoPath = path.join(baseDir, "public", "assets", "brand", "shanghai_karate_logo.webp");
     const qrPath = path.join(baseDir, "public", "assets", "brand", "dojo_qr.png");
 
     const baseImage = sharp(filePath);