fix: share private chat files and harden native lifecycle

Sources/Chat/ChatFeatures.swift

@@ -211,7 +211,8 @@ struct ChatLibraryView: View {
                 Picker("Kind", selection: $kind) { ForEach(["markdown", "text", "code"], id: \.self) { Text($0.capitalized).tag($0) } }
                 TextEditor(text: $content).frame(minHeight: 220)
                 if let artifact {
-                    ShareLink("Export text", item: content)
+                    let file = PrivateChatExport(data: Data(content.utf8), filename: "Hope-artifact-\(artifact.id).\(kind == "markdown" ? "md" : "txt")", type: .plainText)
+                    ShareLink("Export file", item: file, preview: SharePreview(file.filename))
                     if artifact.revision > 1 { Menu("View earlier revision") { ForEach(1..<artifact.revision, id: \.self) { revision in Button("Revision \(revision)") { Task { if let value = await store.historical(artifact, revision: revision) { content = value.content ?? "" } } } } } }
                     Text("Historical content uses the current title and kind. Saving creates a new revision.").font(.footnote)
                 }