Downbit 0.2.6

• downbit mcp keeps its web view past the first video that needs one:
  each script's hidden window was the only window, and closing it quit
  the app's event loop, so later calls in the same process failed and
  calls that ran in parallel crashed. The command line's downloads run
  their scripts the same way, so they're fixed too.
• A script whose window closes gets an error rather than waiting forever.