revert font family for keypad buttons

NotoSansMath appears to be better vertically centered on the keypad than
NotoSans. This is especially noticeable when the font size is increased
in the system settings.

lib/main.dart

@@ -286,6 +286,7 @@ Iterable<A> interleave<A>(Iterable<A> it, A separator) {
 const listPadding = EdgeInsets.all(8.0);
 const _biggerFont = TextStyle(fontSize: 18.0);
 final _biggerFontMath = _biggerFont.copyWith(fontFamily: 'NotoSansMath', fontFamilyFallback: ['NotoSans']);
+final _biggerFontMathButton = _biggerFontMath.copyWith(fontSize: 20.0);  // NotoSansMath appears to be better vertically centered than NotoSans
 
 class KeyboardButton extends StatelessWidget {
   static const _keyIcons = {
@@ -303,7 +304,7 @@ class KeyboardButton extends StatelessWidget {
           height: 44,
           width: 66,
           child: OutlinedButton(
-            child: _keyIcons.containsKey(label) ? Icon(_keyIcons[label], size: 26) : Text(label, style: const TextStyle(fontSize: 20)),
+            child: _keyIcons.containsKey(label) ? Icon(_keyIcons[label], size: 26) : Text(label, style: _biggerFontMathButton),
             onPressed: () => game.keyPressed(label),
           ),
         ),