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.

public/sw.js

@@ -280,3 +280,53 @@ self.addEventListener('message', (event) => {
     }
   }
 });
+
+// Push notification event - receive and display notifications
+self.addEventListener('push', (event) => {
+  if (!event.data) {
+    return;
+  }
+
+  try {
+    const data = event.data.json();
+    const { title, body, icon, badge, tag, data: notificationData } = data;
+
+    const options = {
+      body,
+      icon: icon || '/icon-192.svg',
+      badge: badge || '/icon-192.svg',
+      tag: tag || 'default',
+      data: notificationData,
+      vibrate: [200, 100, 200],
+      requireInteraction: false,
+    };
+
+    event.waitUntil(
+      self.registration.showNotification(title, options)
+    );
+  } catch (error) {
+    console.error('Failed to show notification:', error);
+  }
+});
+
+// Notification click event - handle user interaction
+self.addEventListener('notificationclick', (event) => {
+  event.notification.close();
+
+  const urlToOpen = event.notification.data?.url || '/';
+
+  event.waitUntil(
+    clients.matchAll({ type: 'window', includeUncontrolled: true }).then((clientList) => {
+      // Check if there's already a window open
+      for (const client of clientList) {
+        if (client.url === urlToOpen && 'focus' in client) {
+          return client.focus();
+        }
+      }
+      // Open a new window if none exists
+      if (clients.openWindow) {
+        return clients.openWindow(urlToOpen);
+      }
+    })
+  );
+});