fix: load changelog via DefaultAssetBundle for testability (#214)

Use DefaultAssetBundle.of(context) instead of rootBundle so the asset
bundle can be injected in tests. Add changelog_screen_test.dart with a
FakeAssetBundle that verifies content is shown and that missing-asset
errors are reported correctly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/ui/screens/changelog_screen.dart

@@ -1,7 +1,6 @@
 import 'dart:async';
 
 import 'package:flutter/material.dart';
-import 'package:flutter/services.dart' show rootBundle;
 import 'package:flutter_markdown_plus/flutter_markdown_plus.dart';
 import 'package:url_launcher/url_launcher.dart';
 
@@ -13,7 +12,8 @@ class ChangeLogScreen extends StatelessWidget {
     return Scaffold(
       appBar: AppBar(title: const Text('ChangeLog')),
       body: FutureBuilder<String>(
-        future: rootBundle.loadString('assets/changelog.txt'),
+        future:
+            DefaultAssetBundle.of(context).loadString('assets/changelog.txt'),
         builder: (context, snapshot) {
           if (snapshot.connectionState == ConnectionState.waiting) {
             return const Center(child: CircularProgressIndicator());

test/widget/changelog_screen_test.dart

@@ -0,0 +1,54 @@
+import 'dart:convert';
+
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:sharedinbox/ui/screens/changelog_screen.dart';
+
+class _FakeAssetBundle extends CachingAssetBundle {
+  final Map<String, String> _assets;
+  _FakeAssetBundle(this._assets);
+
+  @override
+  Future<ByteData> load(String key) async {
+    if (_assets.containsKey(key)) {
+      final encoded = utf8.encode(_assets[key]!);
+      return ByteData.view(Uint8List.fromList(encoded).buffer);
+    }
+    throw FlutterError('Asset not found: "$key"');
+  }
+}
+
+const _fakeChangelog =
+    '* 2024-01-01 feat: initial release\n* 2024-01-02 fix: resolve crash\n';
+
+void main() {
+  testWidgets('ChangeLogScreen shows changelog content', (tester) async {
+    await tester.pumpWidget(
+      DefaultAssetBundle(
+        bundle: _FakeAssetBundle({'assets/changelog.txt': _fakeChangelog}),
+        child: const MaterialApp(home: ChangeLogScreen()),
+      ),
+    );
+    await tester.pumpAndSettle();
+
+    expect(find.text('ChangeLog'), findsOneWidget);
+    expect(find.textContaining('initial release'), findsOneWidget);
+    expect(find.textContaining('resolve crash'), findsOneWidget);
+    expect(find.textContaining('Error loading changelog'), findsNothing);
+  });
+
+  testWidgets('ChangeLogScreen shows error when asset is missing', (
+    tester,
+  ) async {
+    await tester.pumpWidget(
+      DefaultAssetBundle(
+        bundle: _FakeAssetBundle({}),
+        child: const MaterialApp(home: ChangeLogScreen()),
+      ),
+    );
+    await tester.pumpAndSettle();
+
+    expect(find.textContaining('Error loading changelog'), findsOneWidget);
+  });
+}