fix(ios): focus close-range barcodes on iPhone Pro cameras

[masanggil1986]

Problem

On iPhone Pro models the rear wide-angle camera has a longer minimum focus distance (~15–20 cm) than the distance at which users naturally hold a barcode. The iOS scanner hard-codes AVCaptureDevice.DiscoverySession(deviceTypes: [.builtInWideAngleCamera], …), so codes held close never come into focus and can't be scanned. Non-Pro iPhones focus closer, so they're unaffected — which is why this only reproduces on Pro devices.

Fix

• Prefer a virtual multi-camera device for the rear position (.builtInTripleCamera → .builtInDualWideCamera → .builtInWideAngleCamera). Virtual devices let iOS automatically switch to the ultra-wide lens for macro focus when the subject is closer than the wide lens can handle.
• Keep the familiar ~1× framing by setting videoZoomFactor to the device's first virtualDeviceSwitchOverVideoZoomFactors value (instead of the 0.5× ultra-wide view), while still allowing the automatic macro switch for close subjects.
• Bias autofocus toward close subjects: autoFocusRangeRestriction = .near + .continuousAutoFocus.
• Focus is configured after startRunning() — starting a session can change the active format, which resets videoZoomFactor — and re-applied when switching back to the rear camera.

Camera selection is shared by the initial setup and the switch-camera button via preferredBackCamera(); focus configuration lives in enableCloseFocus(on:).

Backward compatibility

• No public API change — no Dart or method-channel surface is touched, so existing callers are unaffected.
• Every setting is capability-gated (#available(iOS 13.0, *), isAutoFocusRangeRestrictionSupported, isFocusModeSupported, empty virtualDeviceSwitchOverVideoZoomFactors), so single-lens devices (e.g. iPhone SE) keep the exact previous behavior.
• Deployment target stays iOS 12.0; iOS-13-only APIs are behind #available.

Testing

• ✅ Compiles cleanly — verified by building the bundled example app for the iOS simulator (Xcode 26.6).
• ⏳ Physical-device verification pending — opening as a draft. I'll test on an iPhone Pro shortly, post the results here, and mark the PR ready for review once confirmed. I noticed the README mentions iOS testing is currently a gap — I have Pro hardware available and I'm happy to help test related iOS changes as well.

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud