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/subscribe/route.ts

@@ -0,0 +1,33 @@
+import { NextRequest, NextResponse } from 'next/server';
+import { getServerSession } from 'next-auth';
+import { authOptions } from '@/pages/api/auth/[...nextauth]';
+import { subscribeToPush } from '@/lib/push-notifications';
+
+export async function POST(request: NextRequest) {
+  try {
+    const session = await getServerSession(authOptions);
+
+    if (!session?.user?.id) {
+      return NextResponse.json({ error: 'Unauthorized' }, { status: 401 });
+    }
+
+    const subscription = await request.json();
+
+    if (!subscription.endpoint || !subscription.keys) {
+      return NextResponse.json(
+        { error: 'Invalid subscription object' },
+        { status: 400 }
+      );
+    }
+
+    await subscribeToPush(session.user.id, subscription);
+
+    return NextResponse.json({ success: true });
+  } catch (error) {
+    console.error('Failed to subscribe to push:', error);
+    return NextResponse.json(
+      { error: 'Failed to subscribe to push notifications' },
+      { status: 500 }
+    );
+  }
+}

app/api/push/unsubscribe/route.ts

@@ -0,0 +1,33 @@
+import { NextRequest, NextResponse } from 'next/server';
+import { getServerSession } from 'next-auth';
+import { authOptions } from '@/pages/api/auth/[...nextauth]';
+import { unsubscribeFromPush } from '@/lib/push-notifications';
+
+export async function POST(request: NextRequest) {
+  try {
+    const session = await getServerSession(authOptions);
+
+    if (!session?.user?.id) {
+      return NextResponse.json({ error: 'Unauthorized' }, { status: 401 });
+    }
+
+    const { endpoint } = await request.json();
+
+    if (!endpoint) {
+      return NextResponse.json(
+        { error: 'Endpoint required' },
+        { status: 400 }
+      );
+    }
+
+    await unsubscribeFromPush(session.user.id, endpoint);
+
+    return NextResponse.json({ success: true });
+  } catch (error) {
+    console.error('Failed to unsubscribe from push:', error);
+    return NextResponse.json(
+      { error: 'Failed to unsubscribe from push notifications' },
+      { status: 500 }
+    );
+  }
+}

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 }
+    );
+  }
+}