Reads REPLAYGAIN_TRACK_GAIN (album gain fallback) from FLAC/MP3/M4A tags
and converts Opus R128_TRACK_GAIN/R128_ALBUM_GAIN (Q7.8, -23 LUFS) to the
same ReplayGain dB representation on the Go side, then attenuates the
internal player's volume per track. Off by default; toggle lives under
Library > Playback. Opus header output gain is left to the decoder, which
already applies it.
Closes#465