fix: Re-add oauth2 to serialized structs (#8464)

fix https://github.com/chatmail/core/issues/8463

Followup to https://github.com/chatmail/core/pull/8431: Re-dd `oauth2`
to the two structs that serialized and sent over the wire. This solves
an incompatibility problem where transports could not be synced to older
versions of DC, and profiles could not be transferred, because the
deserializer expected the field to be present.

I now added `#[serde(default)]`, so that with this PR, Delta Chat will
be compatible with both v2.56 and with older versions.

src/transport.rs

@@ -212,6 +212,10 @@ pub(crate) struct ConfiguredLoginParamJson {
     pub smtp_password: String,
 
     pub certificate_checks: ConfiguredCertificateChecks,
+
+    /// Deprecated 2026-07, always false
+    #[serde(default)]
+    pub oauth2: bool,
 }
 
 impl fmt::Display for ConfiguredLoginParam {
@@ -488,6 +492,7 @@ impl From<ConfiguredLoginParam> for ConfiguredLoginParamJson {
             smtp_password: configured_login_param.smtp_password,
 
             certificate_checks: configured_login_param.certificate_checks,
+            oauth2: false,
         }
     }
 }