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>

MarkdownViewer/MarkdownViewerApp.swift

@@ -2,6 +2,11 @@ import SwiftUI
 import UniformTypeIdentifiers
 import AppKit
 
+extension Notification.Name {
+    static let saveDocument = Notification.Name("saveDocument")
+    static let saveDocumentAs = Notification.Name("saveDocumentAs")
+}
+
 @main
 struct MarkdownViewerApp: App {
     @StateObject private var recentFilesManager = RecentFilesManager.shared
@@ -19,6 +24,18 @@ struct MarkdownViewerApp: App {
                 
                 Divider()
                 
+                Button("Save") {
+                    NotificationCenter.default.post(name: .saveDocument, object: nil)
+                }
+                .keyboardShortcut("s", modifiers: .command)
+                
+                Button("Save As...") {
+                    NotificationCenter.default.post(name: .saveDocumentAs, object: nil)
+                }
+                .keyboardShortcut("s", modifiers: [.command, .shift])
+                
+                Divider()
+                
                 Menu("Recent Files") {
                     if recentFilesManager.recentFiles.isEmpty {
                         Text("No Recent Files")
@@ -91,7 +108,8 @@ struct MarkdownViewerApp: App {
             newWindow.contentView = NSHostingView(
                 rootView: MarkdownDocumentView(
                     content: content,
-                    fileName: url.lastPathComponent
+                    fileName: url.lastPathComponent,
+                    fileURL: url
                 )
             )
             newWindow.center()