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>

test/support/fake_app.dart

@@ -19,9 +19,9 @@ class FakeFeeds extends FeedService {
 
   @override
   Future<FeedResult> fetchAll(List<Source> sources,
-      {required Duration Function(Source source) maxAgeOf, String corsProxy = ''}) {
+      {required Duration Function(Source source) windowOf, String corsProxy = ''}) {
     refreshes++;
-    return super.fetchAll(sources, maxAgeOf: maxAgeOf, corsProxy: corsProxy);
+    return super.fetchAll(sources, windowOf: windowOf, corsProxy: corsProxy);
   }
 
   @override