Feature Request: Notifications for Device Online/Offline Status

### What are you trying to do?

I would like to request a feature that sends notifications when a device in my Tailnet goes online or offline. This functionality would be highly beneficial for monitoring critical devices and ensuring smooth operations.

It would be great if these notifications could:

Work seamlessly on both the Tailscale mobile app and desktop client.
Include configurable options (e.g., choose which devices trigger notifications or customize alert settings).
This feature would greatly enhance visibility and usability for Tailnet users, especially in environments where device connectivity is crucial. It would also make me appreciate and enjoy the service even more, knowing I have better control and monitoring capabilities.

Thank you for considering this request!

### How should we solve this?

To implement this feature, Tailscale could introduce a notification system that integrates seamlessly into both the mobile app and desktop client. The system could monitor device connectivity on the Tailnet and send real-time notifications when a device goes online or offline.

Key aspects of the solution could include:

Cross-Platform Notifications: Push notifications for both the mobile app and desktop client.
Customizable Options: Allow users to configure which devices should trigger notifications or set custom rules for alerts (e.g., only for specific groups or during specific times).
API Integration: Provide API endpoints for advanced users to integrate these notifications into third-party tools or services.
Opt-In Feature: Ensure notifications are opt-in to avoid overwhelming users who don’t require them.
By enabling this, Tailscale would provide users with improved control and monitoring, enhancing the overall usability of the service.

Let me know if this aligns with your expectations!

### What is the impact of not solving this?

Reduced Monitoring Efficiency:
Users must manually check the admin console or apps to monitor device statuses, which is time-consuming and inefficient, especially for critical environments.

Missed Critical Events:
Without real-time notifications, users might miss when important devices go offline, potentially leading to disruptions in workflows, security risks, or delays in troubleshooting.

Limited Appeal for Proactive Users:
Users who require robust monitoring might turn to alternative services that offer built-in notification features, reducing Tailscale's appeal for professionals managing critical networks.

Suboptimal User Experience:
A lack of such a feature could result in frustration for users who expect modern tools to provide real-time insights and alerts for improved control over their network.

By addressing this gap, Tailscale can enhance its usability and provide significant value to users managing critical or dynamic environments.

### Anything else?

Increased Manual Effort:
Users would need to frequently monitor their Tailnet manually, increasing the likelihood of human error and potentially overlooking crucial device connectivity changes.

Hindered Scalability for Large Networks:
As Tailnets grow, keeping track of device connectivity becomes more challenging without automated notifications, limiting Tailscale's effectiveness for large teams or enterprises.

Missed Opportunity for Engagement:
Notifications could enhance user engagement with Tailscale’s apps, keeping users connected to their Tailnet and more actively using Tailscale’s ecosystem.

User Perception of Incomplete Features:
Users might perceive the lack of real-time notifications as a gap in functionality, especially when compared to other network management tools that offer similar features.

Addressing this would not only resolve a pain point for existing users but also position Tailscale as a more complete and proactive solution for modern network management needs.