- redesign UI with modern card-based layout

- add dark/light theme toggle functionality
- implement dice notation parser (e.g., `2d6+3`)
- add preset buttons for common dice types
- create animated visual dice representations
- improve roll history with timestamps
- add modifier controls (addition, subtraction, multiplication, division)
- replace bootstrap with custom CSS variables for theming
- enhance mobile responsiveness

Author: TobiasMue91