Fix encryption key exchange and add persistent identity for favorites

- Implement three-tier key system: ephemeral encryption, ephemeral signing, persistent identity
- Fix signature verification by properly separating KeyAgreement and Signing keys
- Add persistent identity key for favorites that survives app restarts
- Fix Data subscript crashes by converting to byte arrays
- Add proper error handling for key exchange
- Private messages remain fully encrypted with ephemeral keys

bitchat/BitchatApp.swift

@@ -1,17 +1,72 @@
 import SwiftUI
+import UserNotifications
 
 @main
 struct BitchatApp: App {
     @StateObject private var chatViewModel = ChatViewModel()
+    #if os(iOS)
+    @UIApplicationDelegateAdaptor(AppDelegate.self) var appDelegate
+    #endif
+    
+    init() {
+        UNUserNotificationCenter.current().delegate = NotificationDelegate.shared
+    }
     
     var body: some Scene {
         WindowGroup {
             ContentView()
                 .environmentObject(chatViewModel)
+                .onAppear {
+                    NotificationDelegate.shared.chatViewModel = chatViewModel
+                }
         }
         #if os(macOS)
         .windowStyle(.hiddenTitleBar)
         .windowResizability(.contentSize)
         #endif
     }
+}
+
+#if os(iOS)
+class AppDelegate: NSObject, UIApplicationDelegate {
+    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey : Any]? = nil) -> Bool {
+        return true
+    }
+}
+#endif
+
+class NotificationDelegate: NSObject, UNUserNotificationCenterDelegate {
+    static let shared = NotificationDelegate()
+    weak var chatViewModel: ChatViewModel?
+    
+    func userNotificationCenter(_ center: UNUserNotificationCenter, didReceive response: UNNotificationResponse, withCompletionHandler completionHandler: @escaping () -> Void) {
+        let identifier = response.notification.request.identifier
+        
+        // Check if this is a private message notification
+        if identifier.hasPrefix("private-") {
+            // Extract sender from notification title
+            let title = response.notification.request.content.title
+            if let senderName = title.replacingOccurrences(of: "Private message from ", with: "").nilIfEmpty {
+                // Find peer ID and open chat
+                if let peerID = chatViewModel?.getPeerIDForNickname(senderName) {
+                    DispatchQueue.main.async {
+                        self.chatViewModel?.startPrivateChat(with: peerID)
+                    }
+                }
+            }
+        }
+        
+        completionHandler()
+    }
+    
+    func userNotificationCenter(_ center: UNUserNotificationCenter, willPresent notification: UNNotification, withCompletionHandler completionHandler: @escaping (UNNotificationPresentationOptions) -> Void) {
+        // Show notification even when app is in foreground (for testing)
+        completionHandler([.banner, .sound])
+    }
+}
+
+extension String {
+    var nilIfEmpty: String? {
+        self.isEmpty ? nil : self
+    }
 }