login [nfc]: Express some unconditional padding more transparently

The top of this area will never be in the top inset, since that's
consumed by the app bar. So "minimum 8" really just means always 8.
So, express that more directly and transparently.

lib/widgets/login.dart

@@ -411,8 +411,9 @@ class LoginPageState extends State<LoginPage> {
         bottom: false,
         child: Center(
           child: SingleChildScrollView(
+            padding: const EdgeInsets.only(top: 8),
             child: SafeArea(
-              minimum: const EdgeInsets.symmetric(vertical: 8),
+              minimum: const EdgeInsets.only(bottom: 8),
               // TODO also detect vertical scroll gestures that start on the
               //   left or the right of this box
               child: ConstrainedBox(