Implement push notification system with PWA support

[Smartdevs17]

Description

Build a push notification system using VAPID protocol with service workers for Progressive Web App, delivering real-time payment and dispute notifications to users.

Acceptance Criteria

• VAPID key generation and management
• Push notification subscription management (subscribe/unsubscribe)
• Service worker registration with push event handling
• Notification categories with user preferences
• Click tracking and deep linking from notifications
• Fallback to in-app notifications when push unavailable

Technical Scope

• frontend/service-worker.ts
• frontend/components/PWAInstallPrompt.tsx
• backend/src/services/vapid.ts
• backend/src/services/push.ts
• backend/src/routes/push.ts
• Edge: notification permission revocation, service worker update cycle, notification grouping