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/RecentFilesManager.swift

@@ -0,0 +1,71 @@
+import Foundation
+
+class RecentFilesManager: ObservableObject {
+    static let shared = RecentFilesManager()
+    
+    @Published var recentFiles: [URL] = []
+    
+    private let maxRecentFiles = 10
+    private let userDefaultsKey = "RecentMarkdownFiles"
+    
+    private init() {
+        loadRecentFiles()
+    }
+    
+    func addRecentFile(_ url: URL) {
+        // Remove if already exists to move it to top
+        recentFiles.removeAll { $0 == url }
+        
+        // Add to beginning
+        recentFiles.insert(url, at: 0)
+        
+        // Limit to max count
+        if recentFiles.count > maxRecentFiles {
+            recentFiles = Array(recentFiles.prefix(maxRecentFiles))
+        }
+        
+        saveRecentFiles()
+    }
+    
+    func removeRecentFile(_ url: URL) {
+        recentFiles.removeAll { $0 == url }
+        saveRecentFiles()
+    }
+    
+    func clearRecentFiles() {
+        recentFiles.removeAll()
+        saveRecentFiles()
+    }
+    
+    private func loadRecentFiles() {
+        if let data = UserDefaults.standard.data(forKey: userDefaultsKey),
+           let urls = try? JSONDecoder().decode([URL].self, from: data) {
+            // Filter out files that no longer exist
+            recentFiles = urls.filter { FileManager.default.fileExists(atPath: $0.path) }
+            
+            // Save filtered list back if any files were removed
+            if recentFiles.count != urls.count {
+                saveRecentFiles()
+            }
+        }
+    }
+    
+    private func saveRecentFiles() {
+        if let data = try? JSONEncoder().encode(recentFiles) {
+            UserDefaults.standard.set(data, forKey: userDefaultsKey)
+        }
+    }
+    
+    func getDisplayName(for url: URL) -> String {
+        return url.lastPathComponent
+    }
+    
+    func getRelativePath(for url: URL) -> String {
+        let homeURL = FileManager.default.homeDirectoryForCurrentUser
+        if url.path.hasPrefix(homeURL.path) {
+            let relativePath = String(url.path.dropFirst(homeURL.path.count))
+            return "~" + relativePath
+        }
+        return url.path
+    }
+}