feat: show real neighboring pages during back and forward swipes

app/test/fetch_actor_test.dart

@@ -0,0 +1,39 @@
+import 'package:actors/actors.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:mews/application/actors/fetch_actor.dart';
+
+const page = '''
+<?xml version="1.0" encoding="UTF-8"?>
+<html xmlns="http://www.w3.org/1999/xhtml">
+  <head><title>Parsed</title><meta name="mews-profile" content="0.1" /></head>
+  <body><p>Some content.</p></body>
+</html>
+''';
+
+void main() {
+  test('ParseSource parses without touching the network', () async {
+    final actor = Actor.create(FetchHandler.new);
+    addTearDown(actor.close);
+
+    final result = await actor
+        .send(const ParseSource(url: 'https://example.com/parsed.html', source: page));
+
+    expect(result, isA<FetchPageOk>());
+    final ok = result as FetchPageOk;
+    expect(ok.page.title, 'Parsed');
+    expect(ok.page.isMews, isTrue);
+    expect(ok.notModified, isFalse);
+  });
+
+  test('ParseSource reports broken markup with its source', () async {
+    final actor = Actor.create(FetchHandler.new);
+    addTearDown(actor.close);
+
+    final result = await actor.send(
+        const ParseSource(url: 'https://example.com/broken.html', source: '<p>nope'));
+
+    expect(result, isA<FetchParseError>());
+    expect((result as FetchParseError).source, '<p>nope');
+  });
+}