逃さない！PWAプッシュ通知でリテンション爆上げ

Create a complete PWA push notification implementation demo. Include: 1) Service worker registration with push event listener, 2) VAPID key generation (use hardcoded for demo), 3) Subscribe/unsubscribe UI with button, 4) Send push notification (simulated via fetch to a mock endpoint or self-contained logic), 5) Offline fallback page. Use clean HTML/CSS/JS, no frameworks. Show notification permission request, subscription state display, and a 'Send Test Notification' button. Style as a modern card UI with glassmorphism.