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

assets/audio/notification.mp3

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+# Notification Sound Asset
+# 
+# This file should contain a short, pleasant notification sound in MP3 format.
+# Recommended specifications:
+# - Duration: 1-2 seconds
+# - Format: MP3, 44.1kHz, 128kbps
+# - Volume: Moderate (not too loud)
+# - License: Ensure proper licensing for commercial use
+#
+# You can create this using:
+# 1. Text-to-speech generators
+# 2. Audio editing software
+# 3. Free notification sound libraries
+# 4. AI-generated sounds
+#
+# Example sources:
+# - Freesound.org (CC licensed)
+# - Zapsplat.com
+# - Adobe Audition
+# - Audacity (free)

assets/notification.mp3

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+