Add PWA push notifications

[vayungodara]

Problem

Currently only in-app and email notifications exist. Mobile users miss real-time nudges for pact reminders, streak warnings, and group activity.

What to change

• Register a Service Worker for push notifications
• Implement Web Push API with VAPID keys
• Add notification permission request flow in components/OnboardingModal.js or settings
• Send push notifications for: pact due reminders, streak-about-to-break warnings, nudges from friends, group activity
• Add a notification preferences page to let users control what they receive

Files to modify

• next.config.js (PWA config)
• New: public/sw.js or use next-pwa
• components/OnboardingModal.js or new notification settings component
• Backend API routes for push subscription management

Impact

Significant engagement boost for mobile users.

Effort

HIGH — ~8-12 hours

[vayungodara]

Closing as duplicate — kept the more detailed version.

[vayungodara]

Closing as duplicate of #21