Function start 

pub fn start(
    provider: &str,
    name: String,
    byo: Option<ByoCredentials>,
    wstore: Arc<Store>,
) -> Result<(String, OAuthStatus), String>

Start an OAuth flow. Resolves config + client (gating on “not configured”), creates a session, and spawns the background task that drives it. Returns (session_id, initial_status). The frontend then polls manager().poll.