Add opt-in push notifications for replies and mentions

Is your feature request related to a problem? Please describe.

DEV currently provides email notifications for replies and mentions, but there is no equivalent opt-in push notification experience for users who want more immediate awareness of active discussions.

For users participating in fast-moving comment threads, email can be too slow or easy to miss.

At the same time, sending push notifications to everyone by default could become noisy.

Describe the solution you'd like

Add optional push notifications for events such as:

• Someone replying to my comment.
• Someone replying to my post.
• Someone mentioning me.
• Other important conversation activity.

Push notifications should be explicitly opt-in, allowing users who prefer DEV's current quieter notification behavior to leave them disabled.

The existing email notification behavior could remain unchanged.

Describe alternatives you've considered

• Continuing to rely on email notifications.
• Keeping DEV open in a browser tab.
• Manually checking comment threads for new activity.

These alternatives are workable but do not provide the immediacy of an opt-in system notification.

Additional context

This feature was proposed in my DEV post:

https://dev.to/dj29/5-devto-features-i-wish-existed-3-im-genuinely-relieved-they-dont-3b8n

The intention is specifically to make push notifications optional rather than changing DEV's default notification behavior.