[codex] Harden setup checks and video downloads

[judefluen-coder]

Summary

• Tighten first-run setup checks for Node versions required by the frontend toolchain.
• Treat FFmpeg as a media-capability requirement instead of blocking dependency installation.
• Make authorized yt-dlp downloads more resilient: preserve completed media when yt-dlp returns a warning, isolate thumbnail/subtitle failures, and fall back to single-file formats when FFmpeg is unavailable.
• Add regression tests for download edge cases and setup doctor version checks.

Validation

• npm run test