- stderr from both processes now displayed in console
- streamlink checked for early exit before starting ffmpeg
- RuntimeError shown to user if stream unavailable
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
On Windows, asyncio subprocess stdin can't accept another process's
StreamReader directly. Use a background task to forward data instead.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>