fix: make ChangeLog screen testable with DefaultAssetBundle (#198)

Switch ChangeLogScreen from rootBundle to DefaultAssetBundle.of(context)
so the asset source can be overridden in widget tests. Add
test/widget/changelog_screen_test.dart with a happy-path test (asset loads
and content is rendered) and an error-path test (missing asset shows the
error message).

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 +1,65 @@
+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 Fake implements AssetBundle {
+  final String content;
+  _FakeAssetBundle(this.content);
+
+  @override
+  Future<String> loadString(String key, {bool cache = true}) async {
+    if (key == 'assets/changelog.txt') return content;
+    throw FlutterError('Asset not found: $key');
+  }
+
+  @override
+  Future<ByteData> load(String key) async {
+    throw FlutterError('Asset not found: $key');
+  }
+}
+
+void main() {
+  testWidgets('ChangeLogScreen renders changelog content', (tester) async {
+    const fakeChangelog =
+        '* 2026-01-01: Initial release\n* 2026-01-02: Bug fix';
+
+    await tester.pumpWidget(
+      DefaultAssetBundle(
+        bundle: _FakeAssetBundle(fakeChangelog),
+        child: const MaterialApp(home: ChangeLogScreen()),
+      ),
+    );
+    await tester.pumpAndSettle();
+
+    expect(find.text('ChangeLog'), findsOneWidget);
+    expect(find.textContaining('2026-01-01'), findsOneWidget);
+    expect(find.textContaining('Initial release'), findsOneWidget);
+  });
+
+  testWidgets('ChangeLogScreen shows error when asset is missing', (
+    tester,
+  ) async {
+    await tester.pumpWidget(
+      DefaultAssetBundle(
+        bundle: _BadAssetBundle(),
+        child: const MaterialApp(home: ChangeLogScreen()),
+      ),
+    );
+    await tester.pumpAndSettle();
+
+    expect(find.textContaining('Error loading changelog:'), findsOneWidget);
+  });
+}
+
+class _BadAssetBundle extends Fake implements AssetBundle {
+  @override
+  Future<String> loadString(String key, {bool cache = true}) async {
+    throw FlutterError('Unable to load asset: "$key"');
+  }
+
+  @override
+  Future<ByteData> load(String key) async {
+    throw FlutterError('Unable to load asset: "$key"');
+  }
+}