- complete ui overhaul with responsive card-based design and dark mode support

- add rich text editing toolbar with formatting buttons
- implement live markdown preview panel with syntax highlighting
- replace `clipboard.js` with `navigator.clipboard` API for better cross-browser support
- add template system for common markdown structures
- implement conversion history with local storage persistence
- add markdown customization options (headings, list markers, etc.)
- add drag and drop support for content
- add keyboard shortcuts (ctrl+enter, ctrl+s, ctrl+d)
- add download functionality for `.md` files
- improve error handling with toast notifications
- replace `alert()` calls with non-blocking UI feedback
- refactor JavaScript for better separation of concerns

Author: TobiasMue91