Remove flutter chat (#26)

Had to squash merge manually as Forgejo was erroring

lib/models/content/encryption.dart

@@ -0,0 +1,23 @@
+import "package:freezed_annotation/freezed_annotation.dart";
+import "package:nexus/models/content/content.dart";
+part "encryption.freezed.dart";
+part "encryption.g.dart";
+
+@freezed
+abstract class EncryptionContent extends Content with _$EncryptionContent {
+  EncryptionContent._();
+  factory EncryptionContent({
+    required String algorithm,
+
+    @JsonKey(name: "rotation_period_ms")
+    @Default(604800000)
+    int rotationPeriodMS,
+
+    @JsonKey(name: "rotation_period_msgs")
+    @Default(100)
+    int rotationPeriodMessages,
+  }) = _EncryptionContent;
+
+  factory EncryptionContent.fromJson(Map<String, Object?> json) =>
+      _$EncryptionContentFromJson(json);
+}