Add recent files menu functionality

Implemented comprehensive recent files tracking system:
- Created RecentFilesManager singleton with UserDefaults persistence
- Added Recent Files submenu to File menu with up to 10 recent files
- Automatic file validation that removes deleted files from list
- Smart file tracking that adds files when opened and moves to top
- Clear Recent Files option for user control
- User-friendly display showing filename and relative path
- Error handling for missing files with user notification

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

CLAUDE.md

@@ -40,7 +40,7 @@ This repository contains a macOS markdown viewer application built with SwiftUI.
 - [x] Live preview while editing
 - [ ] Search within document
 - [ ] Zoom in/out functionality
-- [ ] Recent files menu
+- [x] Recent files menu
 - [ ] Drag and drop file support
 - [ ] Support for math equations (LaTeX)
 - [ ] Image paste support