work in progress implementation of:
- mixer with channel strips, faders, pan knobs, level meters
- timeline with ruler, playhead, sections, keyframe tracks
- pattern and progression pickers for drums/chords
- automation lanes and mute tracks
- loop bracket for loop region selection
- export modal placeholder
known issues:
- drum pattern changes don't update audio engine
- timeline keyframes not connected to scheduler
- some UI bugs remain
this is a checkpoint commit for further iteration
