Add live preview while editing feature

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>

CLAUDE.md

@@ -37,7 +37,7 @@ This repository contains a macOS markdown viewer application built with SwiftUI.
 - [ ] Export to PDF feature
 - [ ] Dark mode support
 - [ ] Custom CSS themes
-- [ ] Live preview while editing
+- [x] Live preview while editing
 - [ ] Search within document
 - [ ] Zoom in/out functionality
 - [ ] Recent files menu