refactor: Update pnpm lockfile and improve Svelte frontend components

Commit notes

This commit downgrades the pnpm lockfile version to 6.0 and updates various package versions, including '@types/node' and 'svelte-check'. It also enhances the Svelte frontend by refactoring components such as SequencerHotkeys and StepSequencer to improve keyboard navigation and user interaction. Additionally, it cleans up the app.css file and modifies the StepButton and StepGrid components for better functionality and maintainability.