- Add mute toggle button (Volume2/VolumeX icons) to the custom control bar.
- Add an always-visible inline straight-line volume slider on desktop;
mobile keeps mute toggle only and relies on system volume keys.
- Slider at 0 auto-mutes; unmuting from zero restores volume to 1.
- Sync isMuted/volume state via the video volumechange event.
Verified in browser at /browse?type=video: drag slider updates
video.volume, mute toggle preserves volume across on/off.
