Add the Fun section to the Learn tab with comic-friendly reader view (#3)

The Learn tab gets a Fun section below Learning with the last 3 days of
Fun articles. Fun articles stay out of Today and the Briefing. New curated
Fun sources: xkcd and The Daily WTF (on by default) and r/ProgrammerHumor
top of the day (off).

Reader view for Fun articles shows the picture whole in the body instead
of as a 16:9 banner cropped to fit, and doesn't call a comic without text
a teaser. When the page gives no picture (Reddit blocks reader view), the
feed's image is shown. The extractor now keeps an image's title text as a
caption (xkcd's last joke), images in blockquotes and behind in-page
anchors (The Daily WTF's Error'd screenshots), drops image maps and
license boxes, and on a text tie prefers the page with more blocks.

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

test/support/fake_app.dart

@@ -1,5 +1,6 @@
 import 'package:daily_tech_news/models/article.dart';
 import 'package:daily_tech_news/models/news_source.dart';
+import 'package:daily_tech_news/services/article_extractor.dart';
 import 'package:daily_tech_news/services/feed_service.dart';
 import 'package:daily_tech_news/services/storage.dart';
 import 'package:daily_tech_news/services/summarizer.dart';
@@ -58,11 +59,13 @@ Future<AppState> launchApp(
   Map<String, List<(String, Duration)>> feeds = const {},
   FeedService? feedService,
   Summarizer? summarizer,
+  ArticleExtractor? extractor,
 }) async {
   final state = AppState(await Storage.open(),
       feedService: feedService ?? FakeFeeds(feeds),
       curatedSources: curatedSources,
-      summarizer: summarizer);
+      summarizer: summarizer,
+      extractor: extractor);
   await state.init();
   return state;
 }