[Task] client - Navbar login, signup 버튼구현 - 05.18

### ISSUE
* Group : client
* Type : bug, func, delete
* Detail: fix actions from client/server

### TODO
- [x] Navbar에서 login, signup 버튼구현
- [x] Navbar는 임시적으로 검은색 적용했다가 추후 투명으로 변경예정
~~- [x] 시간되면 axios 로직틀도 구성할예정.~~

### Estimated time
all-day

### Labels
* Estimated time : all-day
* Group : client
* Sprint : Sprint__lollin__
* Urgency : High or Middle or Low

Start by reviewing the Navbar implementation and the client/server action flow described in the issue. Confirm whether the login and signup buttons and temporary black styling are already complete; if work remains, define the exact change and verify the relevant client behavior.

Written by the indexing model from the issue text.