Replace the single CellSkeleton placeholder with three skeleton cells
of varying heights (2.5rem, 5rem, 2rem) to better mimic a real notebook
loading. Staggered animation delays (0/75/150ms) give a subtle wave
effect on the pulse animation.

The loading state infrastructure from #607 handles detection:
- isLoading=true until bootstrap finds cells or first sync message
- CellSkeleton shows when cells.length===0 && isLoading
- Transition to real cells is automatic via useSyncExternalStore