feat: add Japanese language support, three-language subtitles for all demo scenes

src/composables/useI18n.ts

@@ -1,9 +1,10 @@
 import { ref } from 'vue'
 import zh from '@/locales/zh.json'
 import en from '@/locales/en.json'
+import ja from '@/locales/ja.json'
 
-const messages = { zh, en } as const
-type Lang = 'zh' | 'en'
+const messages = { zh, en, ja } as const
+type Lang = 'zh' | 'en' | 'ja'
 
 const currentLang = ref<Lang>(
   (localStorage.getItem('lang') as Lang) || 'zh'