[task] Loading Indicator 작성

### ISSUE
- Group: `client`
-Type: `bug`, `feature`, `delete`
-Detail: 로딩 인디케이터 만들어 두기

### TODO
1. [ ] 이미지보다 css로 구축 가능하면 할 것.
2. [ ] 누가 봐도 로딩중인지 확인이 가능한가

### Estimated time
>Pick one

### `1h`

The issue names no file or test; start by locating the client-side UI entry point and checking how loading states are rendered. Implement a clearly visible loading indicator, preferring CSS over an image where practical, and verify that users can immediately recognize it as loading.

由索引模型根据 Issue 内容生成。