[Client] 회원가입 기능 구현

### ISSUE
***
Group
- [ ] client
- [ ] server
- [ ] sr

Type
- [ ] bug
- [ ] feature
- [ ] add
- [ ] delete

description
> 기본적인 회원가입 기능을 구현합니다.

### TODO
***
1. [ ] job 1
2. [ ] job 2
3. [ ] job 3

### Estimated time
***
> 8h

No files, tests, entry points, or concrete jobs are named. Start by locating the client-side signup area and determine the required registration flow and its server interface; confirm the three TODO items and acceptance criteria before beginning. Done should mean the basic client signup feature works as agreed and is covered by the relevant checks.

由索引模型根据 Issue 内容生成。