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_GUIDE.md

@@ -0,0 +1,97 @@
+# Web Push Notifications Implementation Guide
+
+## Overview
+
+This guide explains how to implement Web Push notifications in your PWA using VAPID (self-hosted, no external service needed).
+
+## Architecture
+
+1. **VAPID Keys**: Generate once, store securely (like JWT secrets)
+2. **Client**: Request permission, subscribe to push notifications
+3. **Database**: Store push subscriptions per user
+4. **Server**: Send notifications when list events occur
+5. **Service Worker**: Receive and display notifications
+
+## Setup Steps
+
+### 1. Generate VAPID Keys
+
+Run this once to generate your VAPID keys:
+
+```bash
+npx web-push generate-vapid-keys
+```
+
+Add to `.env`:
+```
+VAPID_PUBLIC_KEY=your_public_key_here
+VAPID_PRIVATE_KEY=your_private_key_here
+VAPID_SUBJECT=mailto:your-email@example.com
+```
+
+### 2. Install Dependencies
+
+```bash
+npm install web-push
+```
+
+### 3. Database Schema
+
+Add `PushSubscription` model to store user subscriptions (see prisma/schema.prisma changes).
+
+### 4. Client-Side Flow
+
+1. User grants notification permission
+2. Service worker subscribes to push manager
+3. Subscription sent to server and stored in database
+4. Server sends notifications when events occur
+5. Service worker displays notifications
+
+### 5. Notification Triggers
+
+Notifications are sent when:
+- Someone adds/removes an item from a shared list
+- Someone invites you to a list
+- Someone accepts your list invitation
+- Someone removes you from a list
+
+## Testing
+
+### Local Testing
+- Use Chrome DevTools > Application > Service Workers
+- Use Chrome DevTools > Application > Push Messaging to simulate
+
+### iOS Testing
+- Must be installed to home screen
+- iOS 16.4+ required
+- Test on actual device (simulator doesn't support push)
+
+### Android Testing
+- Works in browser and installed PWA
+- Test on actual device for best results
+
+## Privacy & Best Practices
+
+- Always request permission explicitly (don't auto-prompt)
+- Allow users to disable notifications
+- Store subscriptions securely
+- Handle subscription expiration/updates
+- Respect user preferences per list
+
+## Troubleshooting
+
+### Notifications not appearing
+- Check permission status
+- Verify service worker is active
+- Check browser console for errors
+- Ensure VAPID keys are correct
+
+### iOS specific issues
+- Must be installed to home screen
+- Check iOS version (16.4+)
+- Verify manifest.json is correct
+
+### Subscription fails
+- Check VAPID public key matches server
+- Verify service worker scope
+- Check network requests in DevTools