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.

scripts/generate-vapid-keys.js

@@ -0,0 +1,20 @@
+#!/usr/bin/env node
+
+/**
+ * Generate VAPID keys for Web Push notifications
+ * Run: node scripts/generate-vapid-keys.js
+ */
+
+const webpush = require('web-push');
+
+console.log('Generating VAPID keys for Web Push notifications...\n');
+
+const vapidKeys = webpush.generateVAPIDKeys();
+
+console.log('✓ VAPID keys generated successfully!\n');
+console.log('Add these to your .env file:\n');
+console.log('# Web Push Notifications (VAPID)');
+console.log(`VAPID_PUBLIC_KEY=${vapidKeys.publicKey}`);
+console.log(`VAPID_PRIVATE_KEY=${vapidKeys.privateKey}`);
+console.log('VAPID_SUBJECT=mailto:your-email@example.com');
+console.log('\n⚠️  Keep the private key secret! Do not commit it to version control.\n');