feat(conversation): add empty state title for memory conversation

- Add chatEmpty translation key to English i18n file with message "Is there anything I can help you with？"
- Add chatEmpty translation key to Chinese i18n file with message "有什么我可以帮您的吗？"
- Update Chat component empty state to display title using chatEmpty translation instead of only showing subTitle
- Improve empty state UX by providing a welcoming greeting message to users

web/src/i18n/zh.ts

@@ -1523,6 +1523,7 @@ export const zh = {
       deduplication_desc: '去重消歧完成，最终{{count}}个唯一实体'
     },
     memoryConversation: {
+      chatEmpty:'有什么我可以帮您的吗？',
       searchPlaceholder: '输入用户ID...',
       userID: '用户ID',
       testMemoryConversation: '测试记忆对话',