Connect playlist to surface updates

[superposition]

Description

Wire up playlist state to surface rendering.

Tasks

• Create hooks/usePlaylist.ts
• Implement snapshot preloading (load adjacent snapshots)
• Add smooth transitions between snapshots
• Sync playback state with Zustand store
• Handle end-of-playlist (loop or stop)
• Add loading indicator during preload

Acceptance Criteria

• Surface updates smoothly during playback
• No jank when switching snapshots
• Preloading prevents loading delays

Labels

web-app, frontend, timeline