refactor(index.html): enhance cursor engine and audio management

- Introduced private class fields for better encapsulation in GalacticAudio and GridArchitect classes.
- Simplified sound initialization and playback logic using Maps for sound management.
- Improved cursor size handling based on viewport scroll velocity.
- Updated loading screen message for clarity.
- Refactored CSS variables for cursor size and glow effects.

Author: ledihildawan