[FE/Task] 알림 기능

### ISSUE
- Group: `client`
- Type: `feature`
- Detail: 알림 기능 추가

### TODO
1. [ ] 다가오는 일정 알림
2. [ ] 모임에 새 댓글 알림
3. [ ] 커뮤니티 게시글 새 댓글 알림
4. [ ] 생성한 모임에 새로운 참가자 알림

### Estimated time
### `9h`

The issue identifies the client/FE area and four notification cases: upcoming schedules, new group comments, new community-post comments, and new participants in created groups. Start by surveying the client structure and locating the existing schedule, group, community, and participant flows. Done means all four cases notify the intended recipients; no files or tests are named, so the repository must be mapped first.

Written by the indexing model from the issue text.