Add source kinds and the Learn tab with Learning sources (#2)

Every Source now has a kind: News, Learning or Fun. Curated Sources default
to News, and custom Sources persist their kind (ones saved before default
to News). Each kind has its own time window: News follows the 12/24/48 h
setting, Learning reaches back 14 days, Fun 3 days.

A new Learn tab between Briefing and Sources lists Learning articles with
the same paid/sponsored filters and reader view as Today. It is built from
a list of kinds so a Fun section can follow. Today and the Briefing only
use News. ByteByteGo is the first Learning source, on by default.

The home shell names its tabs instead of using hard-coded indexes.

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

test/support/fake_app.dart

@@ -0,0 +1,68 @@
+import 'package:daily_tech_news/models/article.dart';
+import 'package:daily_tech_news/models/news_source.dart';
+import 'package:daily_tech_news/services/feed_service.dart';
+import 'package:daily_tech_news/services/storage.dart';
+import 'package:daily_tech_news/services/summarizer.dart';
+import 'package:daily_tech_news/state/app_state.dart';
+
+/// Serves canned articles per Source instead of the network. The real
+/// [FeedService.fetchAll] still applies the time windows.
+class FakeFeeds extends FeedService {
+  FakeFeeds([this.byFeedUrl = const {}]);
+
+  /// Feed URL -> (title, age) of each article in the feed.
+  final Map<String, List<(String, Duration)>> byFeedUrl;
+
+  /// How often the app refreshed its feeds.
+  int refreshes = 0;
+
+  @override
+  Future<FeedResult> fetchAll(List<NewsSource> sources,
+      {required Duration Function(NewsSource source) maxAgeOf, String corsProxy = ''}) {
+    refreshes++;
+    return super.fetchAll(sources, maxAgeOf: maxAgeOf, corsProxy: corsProxy);
+  }
+
+  @override
+  Future<List<Article>> fetchSource(NewsSource 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)) {
+      return [Article(title: 'Any', link: '${source.feedUrl}#any', source: source)];
+    }
+    return [
+      for (final (title, age) in byFeedUrl[source.feedUrl]!)
+        Article(
+          title: title,
+          link: '${source.feedUrl}#$title',
+          source: source,
+          published: now.subtract(age),
+        ),
+    ];
+  }
+}
+
+NewsSource testSource(String id, SourceKind kind) => NewsSource(
+      id: id,
+      name: id,
+      feedUrl: 'https://example.com/$id.xml',
+      homepage: 'https://example.com',
+      language: SourceLanguage.en,
+      kind: kind,
+      enabledByDefault: true,
+    );
+
+/// Starts the app the way a launch does, against whatever is in preferences.
+Future<AppState> launchApp(
+  List<NewsSource> curatedSources, {
+  Map<String, List<(String, Duration)>> feeds = const {},
+  FeedService? feedService,
+  Summarizer? summarizer,
+}) async {
+  final state = AppState(await Storage.open(),
+      feedService: feedService ?? FakeFeeds(feeds),
+      curatedSources: curatedSources,
+      summarizer: summarizer);
+  await state.init();
+  return state;
+}