feat: implement secure browser notifications system

- Added SecureNotificationManager with cross-browser support (Chrome, Firefox, Safari, Edge)
- Integrated WebRTC message notifications with tab visibility detection
- Implemented XSS protection, URL validation, and rate limiting
- Notifications shown only when chat tab is inactive
- Enforced HTTPS and user gesture requirements

src/scripts/app-boot.js

@@ -1,6 +1,7 @@
 import { EnhancedSecureCryptoUtils } from '../crypto/EnhancedSecureCryptoUtils.js';
 import { EnhancedSecureWebRTCManager } from '../network/EnhancedSecureWebRTCManager.js';
 import { EnhancedSecureFileTransfer } from '../transfer/EnhancedSecureFileTransfer.js';
+import { NotificationIntegration } from '../notifications/NotificationIntegration.js';
 
 // Import UI components (side-effect: they attach themselves to window.*)
 import '../components/ui/Header.jsx';
@@ -16,6 +17,7 @@ import '../components/ui/FileTransfer.jsx';
 window.EnhancedSecureCryptoUtils = EnhancedSecureCryptoUtils;
 window.EnhancedSecureWebRTCManager = EnhancedSecureWebRTCManager;
 window.EnhancedSecureFileTransfer = EnhancedSecureFileTransfer;
+window.NotificationIntegration = NotificationIntegration;
 
 // Mount application once DOM and modules are ready
 const start = () => {