- No PATH fallback for ffmpeg on Windows (where resolves the working
  directory first); only the bundled binary is used
- Delete the partial .mp4 when encoding fails or is cancelled
- Cancel the export when the dialog is disposed mid-export
- Guard _export against re-entry while the file picker is open
- Run post-export cleanup through the root ProviderContainer so
  coordinate-system/drawing-path restoration no longer depends on the
  dialog staying mounted