🐛 [iOS] Deliver camera result after dismissal and encode JPEG off the main thread

[PierreVieira]

Fixes the two iOS camera picker problems reported in #618.

1. Result delivered before the picker dismissal (CameraControllerDelegate)

imagePickerController(_:didFinishPickingMediaWithInfo:) used to invoke onImagePicked before starting dismissViewControllerAnimated, so the continuation resumed (and client code reacted — navigation, closing Compose dialogs, ...) while the modal was still on screen, racing the UIKit dismissal transition. The gallery path doesn't have this issue in practice because the async NSItemProvider loading naturally delays results past the dismissal.

Now both the success and cancel callbacks are invoked from the dismissal completion handler, so openCameraPicker only returns once the picker is fully gone.

2. JPEG encode + file write on the main thread (FileKit.ios.kt)

UIImageJPEGRepresentation(image, 1.0) on a full-resolution photo plus the multi-MB writeToURL ran inside withContext(Dispatchers.Main), freezing the UI right after the capture. The function is now split in two phases: the UIKit part (presentation + delegate) stays on Dispatchers.Main, and the encode/write moved to Dispatchers.IO. Public API unchanged.

:filekit-dialogs:compileKotlinIosSimulatorArm64 passes. The underlying diagnosis (camera result racing the dismissal while the launcher lived inside a Compose Dialog) was confirmed on a physical iPhone 15 (iOS 17, Compose Multiplatform 1.11.1), where an equivalent deliver-after-dismissal + encode-off-main implementation resolves the freezes; this patch itself is compile-tested.