[ HiveTalk/swarm ] Fix JSON error responses in NIP-05 API

- Replace http.Error calls with proper JSON error responses

- Add Content-Type header for all responses

- Create returnError helper function for consistent error format

- Prevents 'JSON.parse: unexpected keyword' errors in frontend

Now all API responses are properly formatted JSON with success/error status

https://github.com/HiveTalk/swarm/commit/943e4c56c54f46fc17a3c87dec734b5da3e3045f