feat(notifications): implement Web Push notifications for payment confirmations

[frankrichard99]

Summary

Implements Web Push notification support so merchants receive instant alerts when payments are confirmed.

What's included

• Opt-in toggle on /settings page with clear states (on/off/unsupported/denied)
• Service worker registration at app load for background delivery
• Push subscription lifecycle: subscribe → register with backend → unsubscribe → remove from backend
• iOS graceful fallback: Detects unsupported browsers and shows helpful message about adding to Home Screen
• Permission handling: Tracks Notification.permission, shows guidance when denied
• Backend integration: Three new endpoints (/push/subscribe, /push/unsubscribe, /push/status)

Files changed

File	Action	Description
lib/api.ts	Modified	Added PushSubscriptionRequest/Response/Status types and registerPushSubscription, unregisterPushSubscription, getPushSubscriptionStatus methods
hooks/use-push-notifications.ts	Created	Hook managing SW registration, permission state, subscribe/unsubscribe, iOS fallback
components/push-notification-toggle.tsx	Created	UI toggle with backend sync, error handling, and state indicators
app/(app)/settings/page.tsx	Modified	Added Notifications section with PushNotificationToggle
public/sw.js	Replaced	Clean custom SW that caches static assets and imports push-handler.js
app/layout.tsx	Modified	Added inline script for early SW registration

Backend contract

The backend should:

1. Accept POST /push/subscribe with { endpoint, p256dh, auth } → store subscription
2. Accept DELETE /push/unsubscribe → remove subscription
3. Return GET /push/status with { enabled: boolean }
4. On payment confirmed webhook, call the push service with payload:

   {
     "title": "Payment received",
     "body": "You received 500 XLM from a customer",
     "url": "/transactions",
     "tag": "aframp-payment"
   }
   
   ## Closes #496