perf(booth): optimize assets and result flow

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

app/api/images/[filename]/route.ts

@@ -41,7 +41,7 @@ export async function GET(
         "Content-Type": contentType,
         "X-Content-Type-Options": "nosniff",
         "Content-Security-Policy": "default-src 'none'; sandbox",
-        "Content-Disposition": `inline; filename="${filename}"`,
+        "Content-Disposition": `${req.nextUrl.searchParams.get("download") === "1" ? "attachment" : "inline"}; filename="${filename}"`,
         "Cache-Control": "public, max-age=31536000, immutable",
       },
     });