fix: disable mail by default, resolve #2404 (#2405)

Author: ellie

crates/atuin-server/src/settings.rs

@@ -104,6 +104,7 @@ impl<DbSettings: DeserializeOwned> Settings<DbSettings> {
             .set_default("metrics.enable", false)?
             .set_default("metrics.host", "127.0.0.1")?
             .set_default("metrics.port", 9001)?
+            .set_default("mail.enable", false)?
             .set_default("tls.enable", false)?
             .set_default("tls.cert_path", "")?
             .set_default("tls.pkey_path", "")?