VPN state changes should send an OS notification #195

Sibling issue to coder/coder-desktop-windows#40

We should send the user a push notification if there's an unexpected Coder Connect state change.

• Send a notification on fatal OS-VPN-level failures (feat: send push notifications on VPN failures #266)
• Send a notification when an existing session fails to dial the coordination endpoint (requires Coder Desktop should indicate when it fails to dial the coordination endpoint coder#18602)