- completely redesign unit converter UI with modern interface

- add dark mode toggle with system preference detection
- implement responsive layout for mobile compatibility
- add conversion history panel with timestamps
- improve number formatting with appropriate precision
- add copy-to-clipboard functionality
- implement real-time conversion without requiring button press
- add visual formula display showing conversion relationships
- create swap button to quickly reverse conversion direction
- store user preferences and history in localStorage
- add notification system for user feedback
- improve accessibility with ARIA attributes and keyboard navigation

Author: TobiasMue91