- completely redesign UI with modern card-based layout

- add dark/light theme toggle with localStorage persistence
- implement drag and drop file upload functionality
- add image preview with file metadata display
- introduce loading indicator during conversion
- add toast notification system for user feedback
- include output format options (full data URL or base64 only)
- improve error handling and validation
- add responsive design for mobile compatibility
- include informational section about base64 encoding

Author: TobiasMue91