refactor: Update Header and SequencerTrack components for improved layout and styling

Commit notes

This commit modifies the Header component to change the background color and update the logo text from "BeatStep" to "Samplestep." Additionally, it adjusts the padding in the main layout and refines the layout of the SequencerTrack component by enhancing the flex properties for better alignment. These changes aim to improve the visual consistency and user experience across the application.