feat: add iOS support and harden receipt scanning

app/ios/ScanReceipts/Features/Scan/DocumentCameraView.swift

@@ -0,0 +1,65 @@
+import SwiftUI
+import VisionKit
+
+/// Wraps `VNDocumentCameraViewController` (the system document-scanning
+/// camera UI) for SwiftUI. Only the first scanned page is used for v1 — good
+/// enough for a single-receipt photo, and the backend already accepts a
+/// single image per `/api/scan` call.
+///
+/// IMPORTANT: `VNDocumentCameraViewController.isSupported` is `false` on the
+/// Simulator and on devices without a usable camera. Callers must check that
+/// before presenting this view and fall back to the photo picker instead —
+/// this type does not check it itself.
+struct DocumentCameraView: UIViewControllerRepresentable {
+    /// Called exactly once with the captured page, or `nil` if the user
+    /// cancelled or the scan failed. Never left uncalled, so the caller never
+    /// hangs waiting on a result.
+    var onComplete: (UIImage?) -> Void
+
+    func makeUIViewController(context: Context) -> VNDocumentCameraViewController {
+        let controller = VNDocumentCameraViewController()
+        controller.delegate = context.coordinator
+        return controller
+    }
+
+    func updateUIViewController(_ uiViewController: VNDocumentCameraViewController, context: Context) {
+        // No dynamic updates needed.
+    }
+
+    func makeCoordinator() -> Coordinator {
+        Coordinator(onComplete: onComplete)
+    }
+
+    final class Coordinator: NSObject, VNDocumentCameraViewControllerDelegate {
+        private let onComplete: (UIImage?) -> Void
+
+        init(onComplete: @escaping (UIImage?) -> Void) {
+            self.onComplete = onComplete
+        }
+
+        func documentCameraViewController(
+            _ controller: VNDocumentCameraViewController,
+            didFinishWith scan: VNDocumentCameraScan
+        ) {
+            let image = scan.pageCount > 0 ? scan.imageOfPage(at: 0) : nil
+            controller.dismiss(animated: true) {
+                self.onComplete(image)
+            }
+        }
+
+        func documentCameraViewControllerDidCancel(_ controller: VNDocumentCameraViewController) {
+            controller.dismiss(animated: true) {
+                self.onComplete(nil)
+            }
+        }
+
+        func documentCameraViewController(
+            _ controller: VNDocumentCameraViewController,
+            didFailWithError error: Error
+        ) {
+            controller.dismiss(animated: true) {
+                self.onComplete(nil)
+            }
+        }
+    }
+}