- complete redesign of PDF merger UI with modern responsive layout

- add dark mode support with theme toggle and localStorage persistence
- implement file thumbnails for better visual representation
- add file reordering functionality with up/down controls
- improve error handling with styled message notifications
- add loading overlay during merge operations
- implement file validation (size limits, format checking)
- replace jQuery dependencies with vanilla JavaScript
- add accessibility improvements with ARIA labels
- enhance mobile responsiveness with media queries

Author: TobiasMue91