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/ContentView.swift

@@ -1,5 +1,6 @@
 import SwiftUI
 import AppKit
+import UniformTypeIdentifiers
 
 struct ContentView: View {
     var body: some View {
@@ -23,14 +24,18 @@ struct ContentView: View {
 }
 
 struct MarkdownDocumentView: View {
-    let content: String
+    let originalContent: String
     let fileName: String
+    let fileURL: URL?
     @State private var isEditMode = false
     @State private var editableContent: String
+    @State private var hasUnsavedChanges = false
+    @State private var isSaving = false
     
-    init(content: String, fileName: String) {
-        self.content = content
+    init(content: String, fileName: String, fileURL: URL? = nil) {
+        self.originalContent = content
         self.fileName = fileName
+        self.fileURL = fileURL
         self._editableContent = State(initialValue: content)
         print("MarkdownDocumentView: Initialized with content length: \(content.count)")
     }
@@ -49,12 +54,6 @@ struct MarkdownDocumentView: View {
                 
                 Spacer()
                 
-                if isEditMode {
-                    Button("Save") {
-                        // TODO: Implement save functionality
-                    }
-                    .buttonStyle(.borderedProminent)
-                }
             }
             .padding()
             .background(Color(NSColor.controlBackgroundColor))
@@ -91,7 +90,80 @@ struct MarkdownDocumentView: View {
                     .frame(maxWidth: .infinity, maxHeight: .infinity)
             }
         }
-        .navigationTitle(fileName)
+        .navigationTitle(hasUnsavedChanges ? "\(fileName) • Edited" : fileName)
+        .onChange(of: editableContent) { newValue in
+            hasUnsavedChanges = (newValue != originalContent)
+        }
+        .onReceive(NotificationCenter.default.publisher(for: .saveDocument)) { _ in
+            saveFile()
+        }
+        .onReceive(NotificationCenter.default.publisher(for: .saveDocumentAs)) { _ in
+            saveAsNewFile()
+        }
+    }
+    
+    func saveFile() {
+        guard let fileURL = fileURL else {
+            // If no URL, show save dialog
+            saveAsNewFile()
+            return
+        }
+        
+        guard hasUnsavedChanges else { return }
+        
+        isSaving = true
+        
+        Task {
+            do {
+                try editableContent.write(to: fileURL, atomically: true, encoding: .utf8)
+                
+                await MainActor.run {
+                    hasUnsavedChanges = false
+                    isSaving = false
+                }
+            } catch {
+                await MainActor.run {
+                    isSaving = false
+                    showSaveError(error)
+                }
+            }
+        }
+    }
+    
+    func saveAsNewFile() {
+        let panel = NSSavePanel()
+        if let mdType = UTType(filenameExtension: "md") {
+            panel.allowedContentTypes = [mdType]
+        } else {
+            panel.allowedContentTypes = [.plainText]
+        }
+        panel.nameFieldStringValue = fileName
+        
+        if panel.runModal() == .OK, let url = panel.url {
+            isSaving = true
+            
+            Task {
+                do {
+                    try editableContent.write(to: url, atomically: true, encoding: .utf8)
+                    
+                    await MainActor.run {
+                        hasUnsavedChanges = false
+                        isSaving = false
+                    }
+                } catch {
+                    await MainActor.run {
+                        isSaving = false
+                        showSaveError(error)
+                    }
+                }
+            }
+        }
+    }
+    
+    private func showSaveError(_ error: Error) {
+        // For now, just print the error
+        // In a full implementation, we'd show an alert dialog
+        print("Save error: \(error.localizedDescription)")
     }
 }