- redesign the UI with card-based layout and dark mode support

- add game features: undo, hints, progress bar, and timer
- implement keyboard shortcuts (keys `1-6` for colors, `u` for undo)
- add statistics tracking and achievement system
- refactor code into `GameState` and `UI` classes for better separation
- add sound effects using Web Audio API
- implement persistent storage for game settings and progress
- add toast notifications for achievements and game events
- create settings modal for customizing game experience
- add share functionality for game results

Author: TobiasMue91