perf(booth): optimize assets and result flow

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

config/booth.config.json

@@ -5,12 +5,12 @@
     "backgroundType": "image",
     "backgroundColor": "#0b0714",
     "backgroundVideoUrl": "",
-    "backgroundImageUrl": "/assets/theme/shanghai_dojo_night_bg.jpg",
+    "backgroundImageUrl": "/assets/theme/shanghai_dojo_night_bg.webp",
     "primaryColor": "#ff6d00",
     "accentColor": "#06b6d4",
     "surfaceColor": "#170f29",
     "textColor": "#ffffff",
-    "logoUrl": "/assets/brand/shanghai_karate_logo.png",
+    "logoUrl": "/assets/brand/shanghai_karate_logo.webp",
     "logoWidth": 310
   },
   "camera": {
@@ -127,20 +127,20 @@
       "zh": "扫码保存至手机相册"
     },
     "downloadLabel": {
-      "en": "Save Portrait",
-      "zh": "保存高清照"
+      "en": "Save",
+      "zh": "保存"
     },
     "wechatSaveTip": {
-      "en": "In WeChat: Long press photo above to save directly!",
-      "zh": "微信提示：长按上方图片即可直接保存到相册！"
+      "en": "In WeChat, long-press the photo to save it.",
+      "zh": "微信中长按照片即可保存。"
     },
     "printLabel": {
       "en": "Print Photo",
       "zh": "打印照片"
     },
     "startOverLabel": {
-      "en": "Train Again (Start Over)",
-      "zh": "再拍一张"
+      "en": "Finish",
+      "zh": "完成"
     },
     "instagramShareLabel": {
       "en": "Share Photo",