app: correctly propogate emoji font fallback

Author: rajveermalviya

lib/widgets/app.dart

@@ -12,6 +12,7 @@ class ZulipApp extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     final theme = ThemeData(
+      fontFamilyFallback: const <String>['sans-serif', 'Noto Color Emoji'],
       useMaterial3: false, // TODO(#225) fix things and switch to true
       // This applies Material 3's color system to produce a palette of
       // appropriately matching and contrasting colors for use in a UI.

lib/widgets/message_list.dart

@@ -191,7 +191,7 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
     assert(model != null);
     if (!model!.fetched) return const Center(child: CircularProgressIndicator());
 
-    return DefaultTextStyle(
+    return DefaultTextStyle.merge(
       // TODO figure out text color -- web is supposedly hsl(0deg 0% 20%),
       //   but seems much darker than that
       style: const TextStyle(color: Color.fromRGBO(0, 0, 0, 1)),