login [nfc]: Reorder members of LoginPage

Greg points out, referring to the Flutter style guide at
  https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo#order-other-class-members-in-a-way-that-makes-sense :

#600 (comment)
> I think it's likely helpful to treat [`buildRoute`] for the
> ordering as if it does return the class's type, though. It plays
> pretty much the role of a factory constructor — it's the thing
> that application code, at least, should always use instead of
> calling the actual constructor directly. (Possibly we should even
> make the constructor private when we have a `buildRoute` method
> like this; but that might be annoying in tests.)

lib/widgets/login.dart

@@ -238,13 +238,13 @@ class _AddAccountPageState extends State<AddAccountPage> {
 class LoginPage extends StatefulWidget {
   const LoginPage({super.key, required this.serverSettings});
 
-  final GetServerSettingsResult serverSettings;
-
   static Route<void> buildRoute({required GetServerSettingsResult serverSettings}) {
     return _LoginSequenceRoute(
       page: LoginPage(serverSettings: serverSettings));
   }
 
+  final GetServerSettingsResult serverSettings;
+
   @override
   State<LoginPage> createState() => _LoginPageState();
 }