Add Save to Photos action to attachment context menu

Closes #462

Tusker/Views/Attachments/AttachmentView.swift

@@ -444,6 +444,7 @@ extension AttachmentView: UIContextMenuInteractionDelegate {
             }
         } actionProvider: { [unowned self] _ in
             let itemSource: UIActivityItemSource
+            let itemData: Task<Data?, Never>
             if self.attachment.kind == .image,
                let source {
                 switch source {
@@ -457,20 +458,41 @@ extension AttachmentView: UIContextMenuInteractionDelegate {
                         return nil
                     }
                     itemSource = ImageActivityItemSource(data: imageData, url: url, image: image)
+                    itemData = Task { imageData }
+                    
                 case .gifData(let url, let data, let image):
                     itemSource = ImageActivityItemSource(data: data, url: url, image: image)
+                    itemData = Task { data }
                 }
             } else if self.attachment.kind == .gifv {
                 itemSource = GifvActivityItemSource(asset: AVAsset(url: self.attachment.url), attachment: self.attachment)
+                itemData = Task {
+                    try? await URLSession.shared.data(from: self.attachment.url).0
+                }
             } else {
                 return nil
             }
-            return UIMenu(children: [
+            var actions = [
                 UIAction(title: "Share…", image: UIImage(systemName: "square.and.arrow.up")) { [unowned self] _ in
                     let vc = UIActivityViewController(activityItems: [itemSource], applicationActivities: [SaveToPhotosActivity()])
                     self.delegate?.attachmentViewPresent(vc, animated: true)
                 }
-            ])
+            ]
+            let activity = SaveToPhotosActivity()
+            if activity.canPerform(withActivityItems: [self.attachment.url]) {
+                actions.append(UIAction(title: "Save to Photos", image: UIImage(systemName: "square.and.arrow.down"), handler: { _ in
+                    Task {
+                        guard let itemData = await itemData.value else {
+                            return
+                        }
+                        let tempURL = FileManager.default.temporaryDirectory.appendingPathComponent(self.attachment.url.lastPathComponent)
+                        try itemData.write(to: tempURL)
+                        activity.prepare(withActivityItems: [tempURL])
+                        activity.perform()
+                    }
+                }))
+            }
+            return UIMenu(children: actions)
         }
     }