Open multiple documents per markdown file

MarkdownViewer/MarkdownViewerApp.swift

@@ -1,10 +1,84 @@
 import SwiftUI
+import UniformTypeIdentifiers
+import AppKit
 
 @main
 struct MarkdownViewerApp: App {
     var body: some Scene {
-        WindowGroup {
-            ContentView()
+        Settings {
+            EmptyView()
+        }
+        .commands {
+            CommandGroup(replacing: .newItem) {
+                Button("Open Markdown File...") {
+                    openMarkdownFile()
+                }
+                .keyboardShortcut("o", modifiers: .command)
+            }
+        }
+    }
+    
+    private func openMarkdownFile() {
+        let panel = NSOpenPanel()
+        panel.allowsMultipleSelection = true
+        panel.canChooseDirectories = false
+        panel.canChooseFiles = true
+        panel.allowedContentTypes = [UTType.plainText, UTType(filenameExtension: "md")!]
+        
+        if panel.runModal() == .OK {
+            for url in panel.urls {
+                openNewWindow(for: url)
+            }
+        }
+    }
+    
+    private func openNewWindow(for url: URL) {
+        do {
+            let content = try String(contentsOf: url, encoding: .utf8)
+            let newWindow = NSWindow(
+                contentRect: NSRect(x: 0, y: 0, width: 800, height: 600),
+                styleMask: [.titled, .closable, .resizable, .miniaturizable],
+                backing: .buffered,
+                defer: false
+            )
+            
+            newWindow.title = url.lastPathComponent
+            newWindow.contentView = NSHostingView(
+                rootView: MarkdownDocumentView(
+                    content: content,
+                    fileName: url.lastPathComponent
+                )
+            )
+            newWindow.center()
+            newWindow.makeKeyAndOrderFront(nil)
+            
+        } 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)
         }
     }
 }