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/hooks/useMultiTrack.ts

@@ -35,6 +35,7 @@ export function useMultiTrack() {
           effectChain: t.effectChain || createEffectChain(`${t.name} Effects`), // Restore effect chain or create new
           automation: t.automation || { lanes: [], showAutomation: false }, // Restore automation or create new
           selection: t.selection || null, // Initialize selection
+          showEffects: t.showEffects || false, // Restore showEffects state
         }));
       }
     } catch (error) {
@@ -64,6 +65,7 @@ export function useMultiTrack() {
         recordEnabled: track.recordEnabled,
         collapsed: track.collapsed,
         selected: track.selected,
+        showEffects: track.showEffects, // Save effects panel state
         effectChain: track.effectChain, // Save effect chain
         automation: track.automation, // Save automation data
       }));