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

src/components/LangSwitch.vue

@@ -14,6 +14,10 @@ const { currentLang, setLang, t } = useI18n()
       :class="['lang-btn', { active: currentLang === 'en' }]"
       @click="setLang('en')"
     >English</button>
+    <button
+      :class="['lang-btn', { active: currentLang === 'ja' }]"
+      @click="setLang('ja')"
+    >日本語</button>
   </div>
 </template>
 
@@ -29,6 +33,7 @@ const { currentLang, setLang, t } = useI18n()
   color: #888;
   background: rgba(255, 255, 255, 0.05);
   border: 1px solid rgba(255, 255, 255, 0.1);
+  border-radius: 0;
   cursor: pointer;
   transition: all 0.15s;
 }