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/services/feed_service.dart

@@ -23,15 +23,15 @@ class FeedService {
 
   static const _timeout = Duration(seconds: 20);
 
-  /// Fetches all [sources] in parallel and drops articles older than the age
-  /// [maxAgeOf] gives for their Source. Failing feeds are reported in
+  /// Fetches all [sources] in parallel and drops articles older than the time
+  /// window [windowOf] gives for their Source. Failing feeds are reported in
   /// [FeedResult.errors] instead of failing the whole refresh.
   ///
   /// [corsProxy] is only used on web, where browsers block cross-origin feed
   /// requests. It is prepended to the feed URL, e.g. `https://proxy/?url=`.
   Future<FeedResult> fetchAll(
     List<Source> sources, {
-    required Duration Function(Source source) maxAgeOf,
+    required Duration Function(Source source) windowOf,
     String corsProxy = '',
   }) async {
     final errors = <String, String>{};
@@ -50,7 +50,7 @@ class FeedService {
       for (final (i, list) in results.indexed)
         for (final a in list)
           if ((a.published == null ||
-                  a.published!.isAfter(now.subtract(maxAgeOf(sources[i])))) &&
+                  a.published!.isAfter(now.subtract(windowOf(sources[i])))) &&
               seen.add(a.id))
             a,
     ];