Implement handshake request notifications for pending messages (#321)

- Add handshakeRequest packet type (0x25) to notify recipients about queued messages
- Create HandshakeRequest struct with binary encoding for efficient transmission
- Send handshake requests when messages are queued due to missing session
- Display notifications when someone wants to send messages
- Fix verification persistence bug by adding forceSave on app termination
- Update UI to handle optional encryption icons (hide when no handshake attempted)

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

bitchat/Identity/SecureIdentityStateManager.swift

@@ -371,4 +371,10 @@ class SecureIdentityStateManager {
             return cache.verifiedFingerprints.contains(fingerprint)
         }
     }
+    
+    func getVerifiedFingerprints() -> Set<String> {
+        queue.sync {
+            return cache.verifiedFingerprints
+        }
+    }
 }