Move save functionality to File menu

- Remove save button from application toolbar
- Add Save (⌘S) and Save As (⌘⇧S) menu items to File menu
- Implement notification-based communication between menu and document views
- Add unsaved changes indicator in window titles
- Support both saving existing files and creating new files

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

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

CLAUDE.md

@@ -41,6 +41,7 @@ This repository contains a macOS markdown viewer application built with SwiftUI.
 - [ ] Search within document
 - [ ] Zoom in/out functionality
 - [x] Recent files menu
+- [x] Save functionality via File menu (Save/Save As with ⌘S/⌘⇧S shortcuts)
 - [ ] Drag and drop file support
 - [ ] Support for math equations (LaTeX)
 - [ ] Image paste support