fix(barcode-scanner): dispatch iOS cancel() cleanup to the main thread

.changes/barcode-scanner-ios-cancel-main-thread.md

@@ -0,0 +1,12 @@
+---
+"barcode-scanner": patch
+"barcode-scanner-js": patch
+---
+
+Fix a crash on iOS when `cancel()` is invoked. The pending camera
+teardown (`removeFromSuperview`, webView background/opacity updates)
+was running on the background dispatch queue that Tauri uses to
+deliver plugin commands, which caused UIKit to abort the app with an
+`NSAssertionHandler` failure inside `_didMoveFromWindow:toWindow:`.
+The cancel handler now dispatches the UIKit work to the main thread,
+matching the existing pattern used by `scan()` and `openAppSettings()`.

plugins/barcode-scanner/ios/Sources/BarcodeScannerPlugin.swift

@@ -337,10 +337,11 @@ class BarcodeScannerPlugin: Plugin, AVCaptureMetadataOutputObjectsDelegate {
   }
 
   @objc private func cancel(_ invoke: Invoke) {
-    self.invoke?.reject("cancelled")
-
-    destroy()
-    invoke.resolve()
+    DispatchQueue.main.async { [self] in
+      self.invoke?.reject("cancelled")
+      self.destroy()
+      invoke.resolve()
+    }
   }
 }