Implement playback controls

[superposition]

Description

Build playback controls for the timeline.

Tasks

• Create timeline/PlaybackControls.tsx
• Add play/pause toggle button
• Implement speed selector (0.5x, 1x, 2x, 4x)
• Add step forward/backward buttons
• Implement keyboard shortcuts:
  • Space: play/pause
  • Left/Right: step backward/forward
  • 1-5: jump to events
• Show current playback speed

Acceptance Criteria

• Play/pause works correctly
• Speed changes are smooth
• Keyboard shortcuts work globally

Labels

web-app, frontend, timeline