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.

app/profile/page.tsx

@@ -4,6 +4,7 @@ import { authOptions } from '@/pages/api/auth/[...nextauth]'
 import { getUserProfile } from '@/lib/auth-actions'
 import ProfileForm from '@/components/ProfileForm'
 import PasswordChangeForm from '@/components/PasswordChangeForm'
+import NotificationSettings from '@/components/NotificationSettings'
 import Link from 'next/link'
 
 export const metadata = {
@@ -82,6 +83,11 @@ export default async function ProfilePage() {
           <PasswordChangeForm userId={userProfile.id} />
         </div>
 
+        {/* Notification Settings */}
+        <div className="mb-6">
+          <NotificationSettings />
+        </div>
+
         {/* Account Information */}
         <div className="bg-white rounded-xl shadow-md p-6 sm:p-8">
           <h2 className="text-xl sm:text-2xl font-semibold text-gray-900 mb-4">