feat: implement complete web push notification system

- Add self-hosted push notifications using VAPID (no external services)
- Create PushSubscription model in database
- Implement server-side push logic with web-push library
- Add notification helpers for list events (items, invitations, collaborators)
- Create NotificationSettings UI component for profile page
- Add usePushNotifications React hook for subscription management
- Create API routes for subscription management and VAPID key
- Update service worker with push event handlers
- Integrate notifications into item actions (add, remove, complete)
- Integrate notifications into sharing actions (invite, accept, remove)
- Add VAPID key generation script
- Add comprehensive documentation and setup guides

Notifications are sent when:
- Items are added/removed/completed in shared lists
- Users are invited to lists
- Invitations are accepted
- Collaborators are removed from lists

Works on iOS 16.4+ (when installed to home screen), Android, and desktop browsers.

.env.example

@@ -7,3 +7,9 @@ NEXTAUTH_URL=http://localhost:3000
 # Generate a secure secret with: openssl rand -base64 32
 NEXTAUTH_SECRET=your-secret-key-here-please-change-in-production
 AUTH_SECRET=your-secret-key-here-please-change-in-production
+
+# Web Push Notifications (VAPID)
+# Generate keys with: npm run vapid:generate
+VAPID_PUBLIC_KEY=your-vapid-public-key-here
+VAPID_PRIVATE_KEY=your-vapid-private-key-here
+VAPID_SUBJECT=mailto:your-email@example.com