Fix verification persistence on iOS when app backgrounds (#822)

* Fix verification persistence on iOS when app backgrounds (#785)

Verification status was not being saved when the iOS app entered background
state, causing verified contacts to lose their verification status after the
app was closed. This happened because iOS can terminate backgrounded apps
without calling applicationWillTerminate.

Changes:
- Add saveIdentityState() method to force-save identity data without stopping services
- Call saveIdentityState() when iOS app enters background state
- Refactor applicationWillTerminate() to use new method
- Fix selector name typo (appWillTerminate -> applicationWillTerminate)

The verification data is now properly persisted to encrypted keychain storage
whenever the app backgrounds, ensuring verification status persists across
app launches.

Fixes #785

* Save identity state during verification events

---------

Co-authored-by: jack <jackjackbits@users.noreply.github.com>

bitchat/BitchatApp.swift

@@ -189,6 +189,10 @@ final class AppDelegate: NSObject, UIApplicationDelegate {
     func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey : Any]? = nil) -> Bool {
         return true
     }
+    
+    func applicationWillTerminate(_ application: UIApplication) {
+        chatViewModel?.applicationWillTerminate()
+    }
 }
 #endif