Wire the razor tool into Studio's timeline UI:
- B enters razor mode (crosshair cursor + red vertical guide line)
- Click any clip to split at the click position
- Shift+click splits all clips across every track at that time
- V or Escape exits razor mode
- Toolbar shows selection arrow / scissors toggle
Add useRazorSplit hook for split orchestration (HTML + GSAP mutation).
Add activeTool state to playerStore. Add preview reload after timeline
move/resize operations so the composition re-renders with updated timing.