make hovering a reaction show its shortcode

lib/models/reaction_group.dart

@@ -0,0 +1,10 @@
+import "package:fast_immutable_collections/fast_immutable_collections.dart";
+import "package:freezed_annotation/freezed_annotation.dart";
+
+part "reaction_group.freezed.dart";
+
+@Freezed(toJson: false, fromJson: false)
+class const ReactionGroup({
+  required final IList<String> senders,
+  final String? shortcode,
+}) with _$ReactionGroup;