fix: show Plus button only when effects panel is expanded

- Plus button now conditionally rendered based on track.effectsExpanded
- Matches automation controls behavior
- Cleaner UI when panel is collapsed

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

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

components/tracks/TrackList.tsx

@@ -660,7 +660,8 @@ export function TrackList({
                           ))}
                         </div>
 
-                        {/* Add effect button */}
+                        {/* Add effect button - only visible when expanded */}
+                        {track.effectsExpanded && (
                           <Button
                             variant="ghost"
                             size="icon-sm"
@@ -670,6 +671,7 @@ export function TrackList({
                           >
                             <Plus className="h-3 w-3" />
                           </Button>
+                        )}
 
                         {/* Show/hide toggle */}
                         <Button