The brightness set with the left-edge swipe was lost the moment playback
ended, so every session started back at the system level. A new
"Remember Brightness Level" toggle under Settings > Playback > Gestures
persists the level a swipe settles on and reapplies it when the next
playback starts (and when the app resumes mid-session). Leaving the
player still restores the pre-playback brightness, so the rest of the
app is unaffected. Default off, preserving current behavior.
close#2178
edde746