refactor: update OAuthClient::new() signature to accept base_url parameter
OAuthClient now receives the base URL from the caller (AppState.relay_client().base_url_str()) instead of hardcoding it via RelayClient::base_url().
- Add base_url parameter to OAuthClient::new()
- Update call site in load_home_data() to pass state.relay_client().base_url_str()
- Rename check_relay_health() to ping_relay_health() for clarity
- Pass relay_client to ping_relay_health() instead of creating new client
- Update all call sites to use ping_relay_health(state.relay_client())