refactor: move effects panel from global to per-track

- Added showEffects property to Track type
- Added "E" button with Sparkles icon to toggle per-track effects
- Effects panel now appears below each track when toggled
- Removed global EffectsPanel from AudioEditor
- Updated useMultiTrack to persist showEffects state
- Streamlined workflow: both automation and effects are now per-track

This aligns the UX with professional DAWs like Ableton Live, where
effects and automation are track-scoped rather than global.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

lib/audio/track-utils.ts

@@ -59,6 +59,7 @@ export function createTrack(name?: string, color?: TrackColor): Track {
     },
     collapsed: false,
     selected: false,
+    showEffects: false,
     selection: null,
   };
 }