fix: chat::save_msgs: Interrupt inbox loop to send a sync message

Also fix the `send_sync_msg()` documentation: now it's called from the inbox loop, not SMTP (because
`IMAP APPEND` is used to upload sync messages).

src/chat.rs

@@ -4374,7 +4374,7 @@ pub async fn save_msgs(context: &Context, msg_ids: &[MsgId]) -> Result<()> {
             })
             .await?;
     }
-    context.send_sync_msg().await?;
+    context.scheduler.interrupt_inbox().await;
     Ok(())
 }