Function editor_export 

pub async fn editor_export(
    app: AppHandle,
    project: EditProject,
    format: Option<String>,
) -> Result<String, String>

Export the edited project to an .mp4, streaming editor-export-progress events and honoring editor_export_cancel. Returns the output path.

Runs the compose + encode on the blocking pool so the webview stays responsive; progress is throttled to ~100 events over the export.

Errors

Returns a message if the source can’t be opened, the encoder fails, or the export was cancelled.