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.

PUSH_NOTIFICATIONS_QUICK_START.md

@@ -0,0 +1,82 @@
+# Push Notifications - Quick Start
+
+## 🚀 5-Minute Setup
+
+### 1. Install & Generate Keys
+```bash
+npm install
+npm run vapid:generate
+```
+
+### 2. Add Keys to .env
+```env
+VAPID_PUBLIC_KEY=<copy from output>
+VAPID_PRIVATE_KEY=<copy from output>
+VAPID_SUBJECT=mailto:your-email@example.com
+```
+
+### 3. Update Database
+```bash
+npx prisma db push
+```
+
+### 4. Add to Profile Page
+```typescript
+// app/profile/page.tsx
+import NotificationSettings from '@/components/NotificationSettings';
+
+// Add <NotificationSettings /> to your page
+```
+
+### 5. Send Notifications
+```typescript
+// In your action files
+import { notifyItemAdded } from '@/lib/notification-helpers';
+
+// After creating an item:
+await notifyItemAdded(listId, item.title, session.user.id);
+```
+
+## ✅ Done!
+
+Users can now:
+- Enable notifications in their profile
+- Receive push notifications on all their devices
+- Get notified about list activity even when the app is closed
+
+## 📱 iOS Support
+
+For iOS notifications:
+1. Deploy with HTTPS
+2. User must "Add to Home Screen"
+3. Requires iOS 16.4+
+
+## 🔧 Available Notification Functions
+
+```typescript
+// Item notifications
+notifyItemAdded(listId, itemTitle, userId)
+notifyItemRemoved(listId, itemTitle, userId)
+notifyItemCompleted(listId, itemTitle, userId)
+
+// Invitation notifications
+notifyListInvitation(invitationId, email)
+notifyInvitationAccepted(invitationId)
+
+// Collaboration notifications
+notifyRemovedFromList(userId, listTitle, removedByUserId)
+```
+
+## 📚 Full Documentation
+
+- **PUSH_NOTIFICATIONS_SETUP.md** - Complete setup guide
+- **PUSH_NOTIFICATIONS_INTEGRATION_EXAMPLE.md** - Code examples
+- **PUSH_NOTIFICATIONS_GUIDE.md** - Architecture & testing
+
+## 🎯 Key Features
+
+- ✅ Self-hosted (no external services)
+- ✅ Works on iOS, Android, Desktop
+- ✅ Multiple devices per user
+- ✅ Automatic subscription cleanup
+- ✅ Non-blocking (won't slow down your app)