Remove flutter chat (#26)

Had to squash merge manually as Forgejo was erroring

lib/models/content/reaction.dart

@@ -0,0 +1,18 @@
+import "package:freezed_annotation/freezed_annotation.dart";
+import "package:nexus/models/content/content.dart";
+part "reaction.freezed.dart";
+part "reaction.g.dart";
+
+@freezed
+abstract class ReactionContent extends Content with _$ReactionContent {
+  ReactionContent._();
+  static String? keyJsonFromJson(Map<dynamic, dynamic> json, String key) =>
+      json["m.relates_to"]?["key"];
+
+  factory ReactionContent({
+    @JsonKey(readValue: ReactionContent.keyJsonFromJson) String? key,
+  }) = _ReactionContent;
+
+  factory ReactionContent.fromJson(Map<String, Object?> json) =>
+      _$ReactionContentFromJson(json);
+}