feat: Implement hotkey management for sequencer functionality

Commit notes

This commit introduces a new hotkey management system in `hotkeys.ts`, defining key combinations and their corresponding actions for the sequencer. The `SequencerHotkeys.svelte` component is updated to handle keyboard events for play/stop, navigation, division changes, velocity cycling, and pitch adjustments. Additionally, the `StepButton.svelte` component is refactored for improved pitch control via mouse drag. The main page is also updated to initialize mock tracks effectively. This enhancement improves user interaction and accessibility within the sequencer interface.