refactor: share controller disposal and Plex DTO parsing

lib/mixins/controller_disposer_mixin.dart

@@ -0,0 +1,28 @@
+import 'package:flutter/widgets.dart';
+
+/// Owns [TextEditingController] instances created by a [State] and disposes
+/// them automatically from the state's `dispose` chain.
+mixin ControllerDisposerMixin<T extends StatefulWidget> on State<T> {
+  final List<TextEditingController> _textEditingControllers = [];
+
+  TextEditingController createTextEditingController({String? text, TextEditingValue? value}) {
+    assert(text == null || value == null, 'Provide either text or value, not both.');
+    final controller = value == null ? TextEditingController(text: text) : TextEditingController.fromValue(value);
+    _textEditingControllers.add(controller);
+    return controller;
+  }
+
+  TextEditingController registerTextEditingController(TextEditingController controller) {
+    _textEditingControllers.add(controller);
+    return controller;
+  }
+
+  @override
+  void dispose() {
+    for (final controller in _textEditingControllers.reversed) {
+      controller.dispose();
+    }
+    _textEditingControllers.clear();
+    super.dispose();
+  }
+}