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/Shared/ShareUpload.swift

@@ -16,6 +16,10 @@ enum ShareUpload {
     config.sessionSendsLaunchEvents = true
     config.isDiscretionary = false
     config.waitsForConnectivity = true
+    // Authenticate by the request's own header only (see DavClient.session).
+    config.httpShouldSetCookies = false
+    config.httpCookieAcceptPolicy = .never
+    config.urlCredentialStorage = nil
     return config
   }