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/pwa/install-prompt.js

@@ -96,7 +96,7 @@ class PWAInstallPrompt {
 
     setupEventListeners() {
         window.addEventListener('beforeinstallprompt', (event) => {
-            event.preventDefault();
+            // Don't prevent default - let browser show its own banner
             this.deferredPrompt = event;
 
             if (this.checkInstallationStatus()) {