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 prompt (non-intrusive, contextual)
• Send push notifications for: pact deadlines approaching, streak about to break, group member activity
• Add notification preferences in user settings

Acceptance criteria

• Service worker registered
• Push subscription stored in Supabase
• At least one push notification type working end-to-end
• User can opt in/out of notifications
• Works on mobile Chrome and Safari

Label: feature
Priority: medium