fix: share private chat files and harden native lifecycle

Sources/Chat/ChatExport.swift

@@ -0,0 +1,14 @@
+import SwiftUI
+import UniformTypeIdentifiers
+
+/// Share through the system transfer service, not a persistent app cache.
+/// Named representations carry server bytes unchanged and never expose an origin/token.
+struct PrivateChatExport: Transferable {
+    let data: Data
+    let filename: String
+    let type: UTType
+    static var transferRepresentation: some TransferRepresentation {
+        DataRepresentation(exportedContentType: .data) { value in value.data }
+            .suggestedFileName { value in value.filename }
+    }
+}