Add logic for creating new savefile from legacy savefile if new one isn't present

backend/src/api/handler.rs

@@ -547,7 +547,12 @@ impl ApiMessageHandler {
                     } else {
                         // A savefile in the current format doesn't exist yet.
                         // TODO: Parse it to the new format and save it.
+
+                        let app_id_from_game_id: u64;
+                        let new_path = format!("{app_id_from_game_id}.ron");
+
                         let migrated_settings: crate::persist::FileJson = legacy_file.into();
+                        migrated_settings.save(new_path);
                     }
                 } else if current_settings.is_some() {
                     // ...