add optional in_reply_to field on send for conversation threading

hive-c0re/src/dashboard.rs

@@ -1496,6 +1496,7 @@ async fn post_op_send(State(state): State<AppState>, Form(form): Form<OpSendForm
         from: hive_sh4re::OPERATOR_RECIPIENT.to_owned(),
         to: to.clone(),
         body,
+        in_reply_to: None,
     }) {
         return error_response(&format!("op-send to {to} failed: {e:#}"));
     }