Resolve hw_accel=auto to "videotoolbox" on darwin by probing the
configured ffmpeg (hwaccel + both encoders + smoke encode, cached like
the NVENC probe), and add videotoolbox branches to the transcode arg
builder: -hwaccel videotoolbox decode with software output frames (so
the existing CPU filter graph — scaling, subtitle burn-in — applies
unchanged), h264_videotoolbox/hevc_videotoolbox encoders with
bitrate-capped or constant-quality rate control, and GOP forcing on
segment boundaries. h264 forces 8-bit for browser MSE compat; hevc
keeps the source bit depth for HDR10 passthrough. Adds the enum value
to the admin Playback settings and setup wizard dropdowns.
Closes#438
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
