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/models/source.dart

@@ -11,13 +11,28 @@ enum SourceLanguage {
 /// What a Source's articles are for; see GLOSSARY.md. Each kind has its own
 /// section in the app and its own time window.
 enum SourceKind {
-  news('News'),
-  learning('Learning'),
-  fun('Fun');
+  news('News', feedsBriefing: true),
+  learning('Learning', fixedWindow: Duration(days: 14)),
+  fun('Fun', fixedWindow: Duration(days: 3), pictureLed: true);
 
-  const SourceKind(this.label);
+  const SourceKind(
+    this.label, {
+    this.feedsBriefing = false,
+    this.fixedWindow,
+    this.pictureLed = false,
+  });
 
   final String label;
+
+  /// Only News sources feed the Briefing.
+  final bool feedsBriefing;
+
+  /// How far back articles reach; null where the user sets it (News).
+  final Duration? fixedWindow;
+
+  /// Comics and memes are the picture: the reader shows it whole in the body
+  /// rather than as a cropped banner.
+  final bool pictureLed;
 }
 
 /// A Source of any [kind]; see GLOSSARY.md.