On mobile, entering fullscreen now also locks the screen orientation to
landscape via the Screen Orientation API. Exiting fullscreen unlocks it,
allowing the device to return to portrait. Applied to both the video
player and audio player. Gracefully ignored on browsers that don't
support screen.orientation.lock (e.g. iOS Safari).
Also includes the playlist auto-scroll fix (committed separately).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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