refactor: Use adaptive dialog action

lib/widgets/adaptive_dialog_action.dart

@@ -0,0 +1,28 @@
+import 'package:flutter/cupertino.dart';
+import 'package:flutter/material.dart';
+
+class AdaptiveDialogAction extends StatelessWidget {
+  final VoidCallback? onPressed;
+  final Widget child;
+
+  const AdaptiveDialogAction({
+    super.key,
+    required this.onPressed,
+    required this.child,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    final theme = Theme.of(context);
+    switch (theme.platform) {
+      case TargetPlatform.android:
+      case TargetPlatform.fuchsia:
+      case TargetPlatform.linux:
+      case TargetPlatform.windows:
+        return TextButton(onPressed: onPressed, child: child);
+      case TargetPlatform.iOS:
+      case TargetPlatform.macOS:
+        return CupertinoDialogAction(onPressed: onPressed, child: child);
+    }
+  }
+}

lib/widgets/future_loading_dialog.dart

@@ -6,6 +6,7 @@ import 'package:async/async.dart';
 import 'package:flutter_gen/gen_l10n/l10n.dart';
 
 import 'package:fluffychat/utils/localized_exception_extension.dart';
+import 'package:fluffychat/widgets/adaptive_dialog_action.dart';
 
 /// Displays a loading dialog which reacts to the given [future]. The dialog
 /// will be dismissed and the value will be returned when the future completes.
@@ -120,7 +121,7 @@ class LoadingDialogState<T> extends State<LoadingDialog> {
       actions: exception == null
           ? null
           : [
-              TextButton(
+              AdaptiveDialogAction(
                 onPressed: () => Navigator.of(context).pop<Result<T>>(
                   Result.error(
                     exception,