feat: Integrate Firebase Cloud Messaging (FCM)

[gsiddh]

I've added Firebase Cloud Messaging capabilities to your web application.

Here are the key changes:

• I created public/firebase-messaging-sw.js: This handles background notifications, initializing Firebase and displaying notifications when the app is not in focus.
• I modified src/app.ts:
  • I integrated the FCM SDK to request notification permissions from you.
  • I retrieve and log/display the FCM registration token.
  • I added a handler for foreground messages, displaying them directly in the app.
  • I included a placeholder for the VAPID key, which you must configure.
• I updated index.html:
  • I added new UI elements to display the FCM registration token and any notifications received while the app is in the foreground.

Here are the post-integration steps for you:

• Replace the placeholder VAPID key in src/app.ts with your actual key from the Firebase console.
• Ensure your Firebase project is configured for FCM, including the VAPID key.
• Test by sending messages via the Firebase console.