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>

types/track.ts

@@ -32,6 +32,7 @@ export interface Track {
   // UI state
   collapsed: boolean;
   selected: boolean;
+  showEffects: boolean; // Show/hide per-track effects panel
 
   // Selection (for editing operations)
   selection: Selection | null;