Move to Flutter 3.47, use the improvements that come with this (#64)

Reviewed-on: #64

lib/models/content/topic.dart

@@ -1,11 +1,12 @@
 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 _$TopicContent {
+sealed class TopicContent extends Content with _$TopicContent {
   TopicContent._();
   factory TopicContent({
     required String topic,
@@ -16,24 +17,24 @@ abstract class TopicContent extends Content with _$TopicContent {
       _$TopicContentFromJson(json);
 }
 
-@freezed
-abstract class TopicContentBlock with _$TopicContentBlock {
-  factory TopicContentBlock({
-    @Default(IList.empty())
-    @JsonKey(name: "m.text")
-    IList<TextualRepresentation> representations,
-  }) = _TopicContentBlock;
+@Freezed(toJson: false, fromJson: false)
+@JsonSerializable()
+class const TopicContentBlock({
+  final IList<TextualRepresentation> representations = const IList.empty(),
+}) with _$TopicContentBlock {
+  Map<String, Object?> toJson() => _$TopicContentBlockToJson(this);
 
   factory TopicContentBlock.fromJson(Map<String, Object?> json) =>
       _$TopicContentBlockFromJson(json);
 }
 
-@freezed
-abstract class TextualRepresentation with _$TextualRepresentation {
-  factory TextualRepresentation({
-    required String body,
-    @Default("text/plain") String mimetype,
-  }) = _TextualRepresentation;
+@Freezed(toJson: false, fromJson: false)
+@JsonSerializable()
+class const TextualRepresentation({
+  required final String body,
+  final String mimetype = "text/plain",
+}) with _$TextualRepresentation {
+  Map<String, Object?> toJson() => _$TextualRepresentationToJson(this);
 
   factory TextualRepresentation.fromJson(Map<String, Object?> json) =>
       _$TextualRepresentationFromJson(json);