feat: implement push notification system with VAPID-based PWA push notifications

[Tobiloba0]

#closes #357

What Changed

Implemented a complete push notification system for the Progressive Web App using the VAPID protocol and service workers.

Features Added

• Added VAPID key generation and management service.
• Implemented push notification subscription and unsubscription endpoints.
• Added service worker registration and push event handling.
• Introduced notification categories and user preference management.
• Added notification click tracking and deep-link navigation support.
• Implemented fallback in-app notifications when push notifications are unavailable.
• Added handling for notification permission revocation.
• Improved service worker update lifecycle management.
• Added notification grouping support to reduce notification clutter.

Files Changed

• frontend/service-worker.ts
• frontend/components/PWAInstallPrompt.tsx
• backend/src/services/vapid.ts
• backend/src/services/push.ts
• backend/src/routes/push.ts

Why

This PR enables real-time user engagement through web push notifications, allowing users to receive payment and dispute updates even when the application is not actively open. It improves the overall PWA experience while providing fallback mechanisms for environments where push notifications are unavailable or permissions have been revoked.

Testing

• Verified VAPID key generation and validation.
• Tested notification subscription and unsubscription flows.
• Tested service worker registration and push event handling.
• Verified notification click actions and deep-link navigation.
• Tested notification preference filtering by category.
• Verified fallback in-app notifications when push is unsupported.
• Tested service worker update cycle behavior.
• Verified handling of notification permission revocation.

Related Issues

Closes #357

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