FR: Exit node toggle in Control Center that shows its state

### What are you trying to do?

Toggle my exit node from Control Center, with a control that shows whether it is currently on (like the Low Power Mode toggle).

Similar requests were closed, but they don't cover this:

- #15068 was closed pointing to Shortcuts. Shortcuts can toggle the exit node, but a Shortcut in Control Center is a stateless button: it cannot display whether the exit node is active.
- #16037 was closed with the toggle landing in the Home Screen widget. Great, but Control Center is the surface you can reach from inside any app and from the Lock Screen, which is where "am I still routing through my exit node?" actually comes up.

### How should we solve this?

An exit node ControlWidget (iOS 18 Controls API): filled when an exit node is in use, tap to toggle the last used one. #13546 shipped this for the connection itself; this request is the exit node counterpart.

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

For always-on Tailscale users the status bar VPN icon carries no signal, so the only ways to see exit node state are opening the app or going to the Home Screen widget.

### Anything else?

Happy to have this folded into #13546 if you prefer tracking it there.

Start by reviewing the existing Control Center connection control from #13546 and the iOS 18 Controls API. Add an exit-node control that shows whether the last used exit node is active and toggles it; done means its filled state matches the current exit-node status and tapping changes that state.

Written by the indexing model from the issue text.