switch to nullability package

app/lib/utils/form_validators.dart

@@ -1,5 +1,5 @@
-import 'package:lensai/extensions/nullable.dart';
 import 'package:lensai/utils/uri_parser.dart' as uri_parser;
+import 'package:nullability/nullability.dart';
 
 String? validateUrl(
   String? value, {

app/lib/utils/markdown/image_extractor.dart

@@ -1,5 +1,5 @@
-import 'package:lensai/extensions/nullable.dart';
 import 'package:markdown/markdown.dart';
+import 'package:nullability/nullability.dart';
 import 'package:path/path.dart' as p;
 
 List<Uri> extractImagesFromMarkdown(String markdownText) {

app/lib/utils/ui_helper.dart

@@ -1,5 +1,5 @@
 import 'package:flutter/material.dart';
-import 'package:lensai/extensions/nullable.dart';
+import 'package:nullability/nullability.dart';
 import 'package:url_launcher/url_launcher.dart';
 
 void showErrorMessage(BuildContext context, String message) {