Build Notification Preferences Page

# 🧠 Concept
Allow users to customize alert delivery.

## ⚠️ Problem
Users have different communication preferences.

## 📁 Implementation Scope
- apps/web/app/settings/notifications/

## ✅ Requirements
- Discord settings
- Telegram settings
- Webhook settings

## 🎯 Acceptance Criteria
- [ ] Preferences page available
- [ ] Settings persisted
- [ ] Validation implemented