fix(apple): stop delete confirmation re-presenting on macOS

Confirming a transfer/history deletion flashed the same confirmation alert a
second time before it went away. The destructive button runs confirmDelete
synchronously (setting isDeleting = true), while the alert's isPresented
dismiss binding fires asynchronously and then no-ops because its
`if !isDeleting` guard is already false — leaving the open flag set, so macOS
re-reads the binding as true and re-presents the alert until the async delete
finally clears it.

Close the confirmation flag synchronously in confirmDeleteTransfer /
confirmHistoryDelete so there's no window for re-presentation. Tests assert
the flag clears immediately, before the async delete completes.

apple/VniDrop/Features/Send/SendModel.swift

@@ -207,6 +207,9 @@ final class SendModel: ObservableObject {
 	func confirmDeleteTransfer() {
 		guard let transferId = state.selectedTransferId, !state.isDeleting else { return }
 		state.isDeleting = true
+		// Close synchronously: the alert's dismiss binding runs async and no-ops while
+		// `isDeleting`, which would otherwise leave the flag true and macOS re-present it.
+		state.isDeleteConfirmationOpen = false
 		Task {
 			let result = await repository.delete(transferId: transferId)
 			switch result {