[Client] 마이페이지 생성 및 CRUD 구현

### ISSUE
- Type: `feature`
- Detail: 마이페이지 CSS, 정보 CRUD

### TODO
1. [ ] CSS 작성
2. [ ] 유저 정보 불러오기
3. [ ] 유저 정보 수정하기
4. [ ] 회원탈퇴 기능

### Estimated time
> Pick one
### `3h`

No files, tests, or entry points are named. Start by locating the client-side my-page entry point and existing user-information flows, then trace how user data is loaded and updated. Done means the page has its CSS, can load and edit user information, and supports account deletion.

Written by the indexing model from the issue text.