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>

lib/state/app_state.dart

@@ -34,8 +34,10 @@ class AppState extends ChangeNotifier {
     FeedService? feedService,
     ArticleExtractor? extractor,
     List<NewsSource> curatedSources = defaultSources,
+    Summarizer? summarizer,
   })  : _feeds = feedService ?? FeedService(),
         _curated = curatedSources,
+        _summarizerOverride = summarizer,
         consent = ConsentService(),
         paywall = PaywallService(_storage) {
     _extractor = extractor ?? ArticleExtractor(consent: consent);
@@ -46,6 +48,9 @@ class AppState extends ChangeNotifier {
 
   /// The curated catalog; [defaultSources] outside of tests.
   final List<NewsSource> _curated;
+
+  /// Used instead of the AI set up in [settings]; for tests.
+  final Summarizer? _summarizerOverride;
   late final ArticleExtractor _extractor;
   final PaywallService paywall;
 
@@ -56,7 +61,7 @@ class AppState extends ChangeNotifier {
   List<NewsSource> customSources = [];
   Set<String> enabledSourceIds = {};
 
-  /// Everything the feeds returned; see [visibleArticles] for what is shown.
+  /// Everything the feeds returned; see [articlesOf] for what is shown.
   List<Article> articles = [];
   Map<String, String> feedErrors = {};
   bool loading = false;
@@ -77,10 +82,22 @@ class AppState extends ChangeNotifier {
       (settings.hidePaid && paywall.isPaid(a)) ||
       (settings.hideSponsored && paywall.isSponsored(a));
 
-  /// Articles after the paid/sponsored filters. The feed and the briefing use these.
-  List<Article> get visibleArticles => articles.where((a) => !_isHidden(a)).toList();
+  /// Articles from Sources of [kind] after the paid/sponsored filters. Today
+  /// and the Briefing use News, the Learn tab the other kinds.
+  List<Article> articlesOf(SourceKind kind) =>
+      articles.where((a) => a.source.kind == kind && !_isHidden(a)).toList();
 
-  int get hiddenCount => articles.where(_isHidden).length;
+  /// Articles from Sources of [kind] that the paid/sponsored filters hide.
+  int hiddenCountOf(SourceKind kind) =>
+      articles.where((a) => a.source.kind == kind && _isHidden(a)).length;
+
+  /// How far back articles from Sources of [kind] reach. News follows the
+  /// user's setting; the other kinds age slower.
+  Duration windowOf(SourceKind kind) => switch (kind) {
+        SourceKind.news => Duration(hours: settings.maxAgeHours),
+        SourceKind.learning => const Duration(days: 14),
+        SourceKind.fun => const Duration(days: 3),
+      };
 
   List<NewsSource> get allSources => [..._curated, ...customSources];
 
@@ -102,7 +119,7 @@ class AppState extends ChangeNotifier {
     notifyListeners();
     final result = await _feeds.fetchAll(
       enabledSources,
-      maxAge: Duration(hours: settings.maxAgeHours),
+      maxAgeOf: (source) => windowOf(source.kind),
       corsProxy: settings.corsProxy,
     );
     articles = result.articles;
@@ -148,6 +165,7 @@ class AppState extends ChangeNotifier {
     required String name,
     required String feedUrl,
     required SourceLanguage language,
+    SourceKind kind = SourceKind.news,
   }) async {
     final uri = Uri.tryParse(feedUrl);
     final source = NewsSource(
@@ -156,6 +174,7 @@ class AppState extends ChangeNotifier {
       feedUrl: feedUrl,
       homepage: uri == null ? '' : '${uri.scheme}://${uri.host}',
       language: language,
+      kind: kind,
       isCustom: true,
     );
     try {
@@ -226,7 +245,7 @@ class AppState extends ChangeNotifier {
 
   // --- AI ------------------------------------------------------------------
 
-  Summarizer? get _summarizer => switch (settings.aiProvider) {
+  Summarizer? get _summarizer => _summarizerOverride ?? switch (settings.aiProvider) {
         _ when !settings.aiConfigured => null,
         AiProvider.claude => ClaudeSummarizer(
             apiKey: settings.claudeApiKey, model: settings.claudeModel),
@@ -270,7 +289,7 @@ class AppState extends ChangeNotifier {
   Future<void> generateBriefing() async {
     final summarizer = _summarizer;
     if (summarizer == null || briefingLoading) return;
-    final articles = visibleArticles;
+    final articles = articlesOf(SourceKind.news);
     if (articles.isEmpty) {
       briefingError = 'No articles loaded yet. Pull to refresh the feed first.';
       notifyListeners();