[ feature ] fetching data

### ISSUE
- Group: `client`
- Type: `feature`
- Detail: client

### TODO
1. [ ] fetching data using react-query
2. [ ] use custom hooks

### USAGE
1. [ ] explore - entire list
2. [ ] mypage - filtered list
3. [ ] community - posts
4. [ ] post - comments
5. [ ] login - OAuth
6. [ ] buy/sell
7. [ ] create collection

### Estimated time
>

Start in the client code and trace how the explore, mypage, community, post, login, buy/sell, and collection views currently obtain data. Identify the existing data entry points before planning React Query and custom hooks. Done means the listed client usage is covered by the agreed fetching approach, but the issue does not name files, endpoints, or tests.

Written by the indexing model from the issue text.