Set Content-Type on remote notification requests

The webhook POST sends a JSON body built with serde_json, but never declares
it. reqwest's `body()` only sets the payload, so the request goes out without
a Content-Type header and receivers that dispatch on it have to guess.

The wiki documents the payload as JSON, and the recommended receivers
(IFTTT, Svix, SIGNL4) branch on the content type.

Author: karpovantonme

src/notifications/notify_and_log.rs

@@ -234,6 +234,7 @@ fn send_remote_notification(
             let Ok(response) = client
                 .post(remote_notifications.url())
                 .header("User-agent", format!("{SNIFFNET_LOWERCASE}-{APP_VERSION}"))
+                .header("Content-Type", "application/json")
                 .body(notification.to_json())
                 .send()
                 .await