Keep per-kind facts on SourceKind and the Briefing's input in one place (review items 4, 5, 6, 7)

- SourceKind carries whether it feeds the Briefing, its fixed time window
  (Learning, Fun) and whether reading is picture-led (Fun); windowOf and
  the reader read from it.
- AppState.briefingArticles is the one place that says only News sources
  feed the Briefing; generateBriefing and the Briefing tab use it.
- articlesOf, hiddenCountOf and briefingArticles share one filter.
- FeedService.fetchAll takes windowOf: like AppState.windowOf.

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

lib/state/app_state.dart

@@ -82,22 +82,27 @@ class AppState extends ChangeNotifier {
       (settings.hidePaid && paywall.isPaid(a)) ||
       (settings.hideSponsored && paywall.isSponsored(a));
 
+  /// Articles from Sources whose kind passes [test] that the paid/sponsored
+  /// filters show, or with [hidden], those they hide.
+  Iterable<Article> _articlesWhere(bool Function(SourceKind kind) test,
+          {bool hidden = false}) =>
+      articles.where((a) => test(a.source.kind) && _isHidden(a) == hidden);
+
   /// 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();
+  /// uses News, the Learn tab the other kinds.
+  List<Article> articlesOf(SourceKind kind) => _articlesWhere((k) => k == kind).toList();
 
   /// 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;
+  int hiddenCountOf(SourceKind kind) => _articlesWhere((k) => k == kind, hidden: true).length;
+
+  /// What the Briefing is made from: the shown articles of the kinds that
+  /// feed it (News).
+  List<Article> get briefingArticles => _articlesWhere((k) => k.feedsBriefing).toList();
 
   /// 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),
-      };
+  Duration windowOf(SourceKind kind) =>
+      kind.fixedWindow ?? Duration(hours: settings.maxAgeHours);
 
   List<Source> get allSources => [..._curated, ...customSources];
 
@@ -119,7 +124,7 @@ class AppState extends ChangeNotifier {
     notifyListeners();
     final result = await _feeds.fetchAll(
       enabledSources,
-      maxAgeOf: (source) => windowOf(source.kind),
+      windowOf: (source) => windowOf(source.kind),
       corsProxy: settings.corsProxy,
     );
     articles = result.articles;
@@ -292,7 +297,7 @@ class AppState extends ChangeNotifier {
   Future<void> generateBriefing() async {
     final summarizer = _summarizer;
     if (summarizer == null || briefingLoading) return;
-    final articles = articlesOf(SourceKind.news);
+    final articles = briefingArticles;
     if (articles.isEmpty) {
       briefingError = 'No articles loaded yet. Pull to refresh the feed first.';
       notifyListeners();