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>

MarkdownViewer/MarkdownViewerApp.swift

@@ -4,6 +4,8 @@ import AppKit
 
 @main
 struct MarkdownViewerApp: App {
+    @StateObject private var recentFilesManager = RecentFilesManager.shared
+    
     var body: some Scene {
         Settings {
             EmptyView()
@@ -14,6 +16,34 @@ struct MarkdownViewerApp: App {
                     openMarkdownFile()
                 }
                 .keyboardShortcut("o", modifiers: .command)
+                
+                Divider()
+                
+                Menu("Recent Files") {
+                    if recentFilesManager.recentFiles.isEmpty {
+                        Text("No Recent Files")
+                            .disabled(true)
+                    } else {
+                        ForEach(recentFilesManager.recentFiles, id: \.self) { url in
+                            Button(action: {
+                                openRecentFile(url)
+                            }) {
+                                VStack(alignment: .leading) {
+                                    Text(recentFilesManager.getDisplayName(for: url))
+                                    Text(recentFilesManager.getRelativePath(for: url))
+                                        .font(.caption)
+                                        .foregroundColor(.secondary)
+                                }
+                            }
+                        }
+                        
+                        Divider()
+                        
+                        Button("Clear Recent Files") {
+                            recentFilesManager.clearRecentFiles()
+                        }
+                    }
+                }
             }
         }
     }
@@ -32,9 +62,24 @@ struct MarkdownViewerApp: App {
         }
     }
     
+    private func openRecentFile(_ url: URL) {
+        // Check if file still exists
+        guard FileManager.default.fileExists(atPath: url.path) else {
+            recentFilesManager.removeRecentFile(url)
+            showErrorWindow(message: "File no longer exists", details: url.path)
+            return
+        }
+        
+        openNewWindow(for: url)
+    }
+    
     private func openNewWindow(for url: URL) {
         do {
             let content = try String(contentsOf: url, encoding: .utf8)
+            
+            // Add to recent files
+            recentFilesManager.addRecentFile(url)
+            
             let newWindow = NSWindow(
                 contentRect: NSRect(x: 0, y: 0, width: 800, height: 600),
                 styleMask: [.titled, .closable, .resizable, .miniaturizable],
@@ -54,31 +99,34 @@ struct MarkdownViewerApp: App {
             
         } catch {
             print("Error reading file: \(error)")
-            // Show error in a new window
-            let newWindow = NSWindow(
-                contentRect: NSRect(x: 0, y: 0, width: 400, height: 200),
-                styleMask: [.titled, .closable],
-                backing: .buffered,
-                defer: false
-            )
-            
-            newWindow.title = "Error"
-            newWindow.contentView = NSHostingView(
-                rootView: VStack {
-                    Image(systemName: "exclamationmark.triangle")
-                        .font(.system(size: 40))
-                        .foregroundColor(.red)
-                    Text("Error loading file")
-                        .font(.headline)
-                    Text(error.localizedDescription)
-                        .font(.caption)
-                        .foregroundColor(.secondary)
-                        .multilineTextAlignment(.center)
-                }
-                .padding()
-            )
-            newWindow.center()
-            newWindow.makeKeyAndOrderFront(nil)
+            showErrorWindow(message: "Error loading file", details: error.localizedDescription)
         }
     }
+    
+    private func showErrorWindow(message: String, details: String) {
+        let newWindow = NSWindow(
+            contentRect: NSRect(x: 0, y: 0, width: 400, height: 200),
+            styleMask: [.titled, .closable],
+            backing: .buffered,
+            defer: false
+        )
+        
+        newWindow.title = "Error"
+        newWindow.contentView = NSHostingView(
+            rootView: VStack {
+                Image(systemName: "exclamationmark.triangle")
+                    .font(.system(size: 40))
+                    .foregroundColor(.red)
+                Text(message)
+                    .font(.headline)
+                Text(details)
+                    .font(.caption)
+                    .foregroundColor(.secondary)
+                    .multilineTextAlignment(.center)
+            }
+            .padding()
+        )
+        newWindow.center()
+        newWindow.makeKeyAndOrderFront(nil)
+    }
 }