Remove flutter chat (#26)

Had to squash merge manually as Forgejo was erroring

lib/models/content/topic.dart

@@ -0,0 +1,40 @@
+import "package:fast_immutable_collections/fast_immutable_collections.dart";
+import "package:freezed_annotation/freezed_annotation.dart";
+import "package:nexus/models/content/content.dart";
+part "topic.freezed.dart";
+part "topic.g.dart";
+
+@freezed
+abstract class TopicContent extends Content with _$TopicContent {
+  TopicContent._();
+  factory TopicContent({
+    required String topic,
+    @JsonKey(name: "m.topic") TopicContentBlock? content,
+  }) = _TopicContent;
+
+  factory TopicContent.fromJson(Map<String, Object?> json) =>
+      _$TopicContentFromJson(json);
+}
+
+@freezed
+abstract class TopicContentBlock with _$TopicContentBlock {
+  factory TopicContentBlock({
+    @Default(IList.empty())
+    @JsonKey(name: "m.text")
+    IList<TextualRepresentation> representations,
+  }) = _TopicContentBlock;
+
+  factory TopicContentBlock.fromJson(Map<String, Object?> json) =>
+      _$TopicContentBlockFromJson(json);
+}
+
+@freezed
+abstract class TextualRepresentation with _$TextualRepresentation {
+  factory TextualRepresentation({
+    required String body,
+    @Default("text/plain") String mimetype,
+  }) = _TextualRepresentation;
+
+  factory TextualRepresentation.fromJson(Map<String, Object?> json) =>
+      _$TextualRepresentationFromJson(json);
+}