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/ui/article_screen.dart

@@ -6,7 +6,6 @@ import 'package:flutter/material.dart';
 import 'package:flutter_inappwebview/flutter_inappwebview.dart';
 
 import '../models/article.dart';
-import '../models/source.dart';
 import '../services/article_extractor.dart';
 import '../services/consent_service.dart';
 import '../services/content_blocking.dart';
@@ -276,9 +275,7 @@ class _ReaderView extends StatelessWidget {
         builder: (context, snapshot) {
           final loaded = snapshot.data;
           var blocks = loaded?.blocks ?? const <ReaderBlock>[];
-          // Comics and memes are the picture: show it whole in the body
-          // rather than as a cropped banner.
-          final pictureLed = article.source.kind == SourceKind.fun;
+          final pictureLed = article.source.kind.pictureLed;
           final banner = pictureLed ? null : article.imageUrl;
           // The page's own hero image duplicates the feed image shown on top.
           if (banner != null) {

lib/ui/briefing_screen.dart

@@ -1,7 +1,6 @@
 import 'package:flutter/material.dart';
 
 import '../models/briefing.dart';
-import '../models/source.dart';
 import '../state/app_state.dart';
 import 'article_screen.dart';
 import 'briefing_history_screen.dart';
@@ -60,7 +59,7 @@ class BriefingScreen extends StatelessWidget {
     if (state.briefingLoading) {
       return EmptyState(
         icon: Icons.hourglass_top,
-        title: 'Reading ${state.articlesOf(SourceKind.news).length} headlines…',
+        title: 'Reading ${state.briefingArticles.length} headlines…',
         message: 'This usually takes 10-60 seconds.',
         action: const SizedBox(width: 160, child: LinearProgressIndicator()),
       );
@@ -70,7 +69,7 @@ class BriefingScreen extends StatelessWidget {
       icon: const Icon(Icons.auto_awesome),
       label: Text(state.loading
           ? 'Waiting for feeds…'
-          : 'Generate from ${state.articlesOf(SourceKind.news).length} articles'),
+          : 'Generate from ${state.briefingArticles.length} articles'),
     );
     if (briefing == null) {
       return EmptyState(