회원가입 페이지 목업 구현

### ISSUE
- Group: `client`
- Type: `feature`
- Detail: 회원가입 페이지 목업 구현

### TODO
1. [ ] Job1 입력창 만들기
2. [ ] Job2 유효성검사 메시지 공간 확보
3. [ ] Job3 회원가입 리다이렉트 버튼
4. [ ] Job4 소셜로그인 버튼
5. [ ] Job5 모달창 구현

### Estimated time
> Pick one
### `2h`

### Labels
- Estimated time: `E: 2h`
- Group : `client`
- Sprint: `Sprint__NUMBER__`
- Urgency: `High`

No file, test, or entry point is named. Start by locating the client-side signup page and review the existing page and routing structure; use the five TODO items as the completion checklist, including inputs, validation-message space, redirect and social-login buttons, and the modal.

Written by the indexing model from the issue text.