iOS pairing scanner: don't start the camera until the user taps

[kairan]

Related: #23833

Problem

On iOS, opening Add environment starts the camera on its own whenever camera permission was granted before. Someone who only wants to type a host and pairing code still turns the camera on, and the green privacy indicator comes on with it. The issue in #23833 reports that indicator staying lit after leaving the screen. I could not reproduce that part on an iPhone 14 Pro (iOS 27.0): the dot cleared after leaving. The "Camera, a moment ago" entry in Settings → Privacy is normal after any scan.

Idea

Show the existing "Tap to scan" card every time the screen is entered, and start the camera only after the user taps it. Leaving the screen (or covering it with another one) goes back to that state, so re-entering never starts the camera by itself.

Trade-off

People who have already granted permission lose auto-scan and need one extra tap. In exchange, the camera only runs when the user asked for it. The "Tap to scan" UI already exists, so there is no new design.

Question for maintainers

Is this behavior change wanted? If yes, I'll send a small PR that links this discussion.

[kairan]

Update: I have a working fix in my local environment (tap-to-start on the pairing scanner so the camera does not auto-start on screen entry). I am not opening a PR yet — waiting for maintainer direction here first.

Happy to share a patch or before/after captures if that helps review the approach.