Refactor StepSequencer and SequencerTrack components for improved UI and code clarity

Commit notes

- Removed unused sample management logic from StepSequencer, simplifying the component structure. - Updated icon imports in StepSequencer for consistency with the new design. - Wrapped track rendering in a Paper component for enhanced visual presentation in StepSequencer. - Adjusted class names in SequencerTrack for better styling consistency across light and dark themes. - Cleaned up imports and optimized component code for maintainability.