[프론트엔드] <SideBar> 일반 로그인 기능 구현

## ISSUE
- Group: `client`
- Type: `feature`
- Detail: `Side Bar: 일반 로그인 페이지`

## TODO
- [ ] 페이지 로그인: 토큰요청
post/ {email, password}
- [ ] 토큰 상태 변경
- [ ] 로그인 상태 변경

## Estimated time

### `3hs`

## Labels
Estimated time: E: `3hs`
Group : `client`
Sprint: `Sprint 2`
Urgency: `bare`

No file or test is named. Start by locating the client-side SideBar and existing login or token-state entry points, then confirm the POST endpoint and request shape shown in the issue. Done means the login page requests a token and both token state and login state update successfully.

Written by the indexing model from the issue text.