Implemented split-view editor with real-time markdown preview:
- Added Edit/Preview toggle button with toolbar
- Created MarkdownEditor component using NSTextView for raw markdown editing
- Implemented HSplitView with editor on left and live preview on right
- Fixed NSTextView width constraints to prevent unwanted scaling
- Added proper text container configuration for word wrapping
- Text changes update preview in real-time via SwiftUI binding
🤖 Generated with [Claude Code](https://claude.ai/code)
Co-Authored-By: Claude <noreply@anthropic.com>
