Integrate ReceiptCameraSheet into expense wizard flow

The expense wizard state (BLoC or Riverpod Notifier) should hold a receiptAttachmentServiceId or a reference to the active ReceiptAttachmentService instance so that the ReceiptAttachmentIndicator and the submit logic share the same service instance. Do not reconstruct ReceiptAttachmentService on each build — provide it via dependency injection (Riverpod Provider or BLoC dependency). For the 'wait for upload on submit' behaviour, use await service.uploadCompleter.future inside the form submission handler — ReceiptAttachmentService should expose a Future that resolves when the current upload succeeds or fails. Use showModalBottomSheet with a Future return — the wizard's _onAttachTapped method is async and awaits the sheet result before calling service.startUpload().

Keep the wizard's form key (GlobalKey) stable across rebuilds to prevent state loss.

Widget integration tests: verify form fields retain values after ReceiptCameraSheet dismissal; verify ReceiptAttachmentService.startUpload is called when sheet returns a file; verify ReceiptAttachmentIndicator transitions to uploading state immediately after image selection; verify form submit button is disabled with correct error message when receipt is mandatory and not attached; verify form submit is enabled when amount is below 100 kr regardless of attachment; verify wizard submits successfully when upload completes before submission; verify wizard waits for in-progress upload before submitting. Use mocktail for ReceiptAttachmentService and flutter_test pump helpers to simulate async upload completion. Test on both iOS and Android form factor targets.