Add APP_ID_UNKNOWN sentinel value for required app_id field during savefile migration

backend/src/persist/migration.rs

@@ -1,2 +1,4 @@
 pub mod gpu;
 pub mod settings;
+
+pub const APP_ID_UNKNOWN: u64 = u64::MAX;

backend/src/persist/migration/settings.rs

@@ -51,7 +51,7 @@ impl From<OldSettingsJson> for FileJson {
         Self {
             version: LATEST_VERSION,
             name: old_settings.name.clone(),
-            app_id: todo!(),
+            app_id: super::APP_ID_UNKNOWN, // `u64::MAX`, sentinel value.
             variants,
         }
     }