Captive Portal Integration

### Feature Description

It'd be nice to have a [captive portal](https://support.mozilla.org/en-US/kb/captive-portal) appear directly when connecting to a WiFi network that requires it. I'm proposing a pop-up window similar to the ones in mainstream OSes like iOS, macOS, and Android. This window could use the user's default browser or use a bundled browser engine (I imagine the latter is less likely).

### Use Case

This feature is useful when connecting to (often) public WiFi networks that use captive portals to allow you to use the Internet.

Opening Firefox manually isn't an elegant solution. DMS will show me I'm connected to WiFi, but my terminal apps won't connect to the Internet, so I have to remember that I'm signed into a public network, turn off my VPN, and finally open Firefox. I think DMS could offer a better solution since it knows the system's WiFi status and VPN configuration, so it could automatically present such a pop-up.

### Compositor(s)

All compositors

### If Other, please specify

_No response_

### Proposed Solution

_No response_

### Alternatives/Existing Solutions

_No response_

No files, tests, or entry points are named. Start by locating how DMS observes WiFi and VPN state and how it presents desktop windows, then investigate how captive portals can be detected and opened. Done means a captive portal appears when a qualifying WiFi connection is established, across all listed compositors.

Written by the indexing model from the issue text.