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/api/push/vapid-key/route.ts

@@ -0,0 +1,23 @@
+import { NextResponse } from 'next/server';
+import { getVapidPublicKey } from '@/lib/push-notifications';
+
+export async function GET() {
+  try {
+    const publicKey = await getVapidPublicKey();
+
+    if (!publicKey) {
+      return NextResponse.json(
+        { error: 'Push notifications not configured' },
+        { status: 503 }
+      );
+    }
+
+    return NextResponse.json({ publicKey });
+  } catch (error) {
+    console.error('Failed to get VAPID public key:', error);
+    return NextResponse.json(
+      { error: 'Failed to get VAPID public key' },
+      { status: 500 }
+    );
+  }
+}