- completely redesign base64 utility with modern UI/UX

- add dark mode with automatic system preference detection
- implement file upload functionality for both text and binary files
- add URL-safe base64 option (using -_ instead of +/)
- introduce auto-update toggle for real-time encoding/decoding
- add character counter for both input and output
- implement copy to clipboard and download functionality
- improve error handling with notification system
- ensure proper UTF-8 handling for international text
- make responsive design for mobile compatibility

Author: TobiasMue91