Rename NewsSource to Source now that it models every kind (review item 3)

The class and its file (lib/models/source.dart) follow the glossary term.
Persisted JSON keys are unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/support/fake_app.dart

@@ -1,5 +1,5 @@
 import 'package:daily_tech_news/models/article.dart';
-import 'package:daily_tech_news/models/news_source.dart';
+import 'package:daily_tech_news/models/source.dart';
 import 'package:daily_tech_news/services/article_extractor.dart';
 import 'package:daily_tech_news/services/feed_service.dart';
 import 'package:daily_tech_news/services/storage.dart';
@@ -18,14 +18,14 @@ class FakeFeeds extends FeedService {
   int refreshes = 0;
 
   @override
-  Future<FeedResult> fetchAll(List<NewsSource> sources,
-      {required Duration Function(NewsSource source) maxAgeOf, String corsProxy = ''}) {
+  Future<FeedResult> fetchAll(List<Source> sources,
+      {required Duration Function(Source source) maxAgeOf, String corsProxy = ''}) {
     refreshes++;
     return super.fetchAll(sources, maxAgeOf: maxAgeOf, corsProxy: corsProxy);
   }
 
   @override
-  Future<List<Article>> fetchSource(NewsSource source, {String corsProxy = ''}) async {
+  Future<List<Article>> fetchSource(Source source, {String corsProxy = ''}) async {
     final now = DateTime.now().toUtc();
     // A feed nobody set up still has an article, so it can be added.
     if (!byFeedUrl.containsKey(source.feedUrl)) {
@@ -43,7 +43,7 @@ class FakeFeeds extends FeedService {
   }
 }
 
-NewsSource testSource(String id, SourceKind kind) => NewsSource(
+Source testSource(String id, SourceKind kind) => Source(
       id: id,
       name: id,
       feedUrl: 'https://example.com/$id.xml',
@@ -55,7 +55,7 @@ NewsSource testSource(String id, SourceKind kind) => NewsSource(
 
 /// Starts the app the way a launch does, against whatever is in preferences.
 Future<AppState> launchApp(
-  List<NewsSource> curatedSources, {
+  List<Source> curatedSources, {
   Map<String, List<(String, Duration)>> feeds = const {},
   FeedService? feedService,
   Summarizer? summarizer,