internxt: surface re-login error when re-auth fails in NewFs

When the stored token is expired and refreshOrReLogin fails, the
returned error wrapped the original 401 and discarded authErr, hiding
the actionable reason (e.g. "account requires 2FA - please run: rclone
config reconnect remote:"). Wrap both errors so the user sees why
re-auth failed and what to do about it.

Fixes #9583

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/internxt/internxt.go

@@ -343,7 +343,7 @@ func NewFs(ctx context.Context, name, root string, m configmap.Mapper) (fs.Fs, e
 			fs.Debugf(f, "getUserInfo returned 401, attempting re-auth")
 			authErr := f.refreshOrReLogin(ctx)
 			if authErr != nil {
-				return nil, fmt.Errorf("failed to fetch user info (re-auth failed): %w", err)
+				return nil, fmt.Errorf("failed to fetch user info (re-auth failed: %w): %w", authErr, err)
 			}
 			userInfo, err = getUserInfo(ctx, &userInfoConfig{Token: f.cfg.Token})
 			if err == nil {