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.

package.json

@@ -13,7 +13,8 @@
     "db:studio": "prisma studio",
     "db:migrate": "npx tsx scripts/migrate-lists.ts",
     "icons:generate": "node scripts/generate-icons.js",
-    "icons:info": "node scripts/svg-to-png.js"
+    "icons:info": "node scripts/svg-to-png.js",
+    "vapid:generate": "node scripts/generate-vapid-keys.js"
   },
   "dependencies": {
     "@auth/prisma-adapter": "^2.11.1",
@@ -25,6 +26,7 @@
     "next-auth": "^4.24.5",
     "react": "^18",
     "react-dom": "^18",
+    "web-push": "^3.6.7",
     "zod": "^4.1.13"
   },
   "devDependencies": {
@@ -32,6 +34,7 @@
     "@types/node": "^20",
     "@types/react": "^18",
     "@types/react-dom": "^18",
+    "@types/web-push": "^3.6.3",
     "@vitest/ui": "^4.0.14",
     "autoprefixer": "^10.0.1",
     "eslint": "^8",