Function send_dm 

pub async fn send_dm(
    receiver_npub: &str,
    content: &str,
    reply_to: Option<&str>,
    config: &SendConfig,
    callback: Arc<dyn SendCallback>,
) -> Result<SendResult, String>

Send a NIP-17 gift-wrapped text DM.

Flow: pending msg → callback.on_pending → build Kind 14 rumor → gift-wrap with retry → finalize → callback.on_sent → optional self-send.