fix: share private chat files and harden native lifecycle

Tests/ChatTests.swift

@@ -43,10 +43,13 @@ final class ChatTests: XCTestCase {
         XCTAssertFalse(store.researchAvailable)
         await store.exportConversation(format: "markdown")
         XCTAssertTrue(store.exportText?.contains("Export this") == true)
+        XCTAssertTrue(store.exportFile?.filename.hasSuffix(".md") == true)
+        XCTAssertEqual(store.exportFile?.data, Data(store.exportText!.utf8))
         store.foreground(true)
         store.foreground(false)
         store.close()
         XCTAssertNil(store.exportText)
+        XCTAssertNil(store.exportFile)
         XCTAssertFalse(store.research)
     }
     func testBranchAndRegeneratePreserveOriginal() async throws {