app: set a global font family fallback for emojis

Author: rajveermalviya

lib/widgets/app.dart

@@ -12,6 +12,14 @@ class ZulipApp extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     final theme = ThemeData(
+      // This correctly sets up a the font fallback for normal text that
+      // may contain an emoji, where it will use any font from the "sans-serif"
+      // group to fetch the glyphs and fallback to "Noto Color Emoji" for emojis.
+      //
+      // Note that specifiying only "Noto Color Emoji" in the fallback list,
+      // Flutter tries to use it to draw even the non emoji characters;
+      // which leads to broken text rendering.
+      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.