iOS: authenticate each request by its own header only (no cookies/credential cache)

Two accounts can share one server; the Share Extension's folder listing and
the upload sessions now ignore cookies and stored credentials so one account's
session can never answer for another. Logs each PROPFIND (user, path, status).

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

ios/Runner/Native/TransferManager.swift

@@ -34,6 +34,11 @@ final class TransferManager: NSObject {
     config.sessionSendsLaunchEvents = true
     config.isDiscretionary = false
     config.waitsForConnectivity = true
+    // Two accounts can share a server: authenticate by the request's own
+    // header only, never a cookie another account's request left behind.
+    config.httpShouldSetCookies = false
+    config.httpCookieAcceptPolicy = .never
+    config.urlCredentialStorage = nil
     return URLSession(configuration: config, delegate: self, delegateQueue: nil)
   }()