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>

lib/data/default_sources.dart

@@ -212,6 +212,37 @@ const defaultSources = <NewsSource>[
     description: 'System design explained with diagrams',
     enabledByDefault: true,
   ),
+
+  // --- Fun ----------------------------------------------------------------
+  NewsSource(
+    id: 'xkcd',
+    name: 'xkcd',
+    feedUrl: 'https://xkcd.com/atom.xml',
+    homepage: 'https://xkcd.com',
+    language: SourceLanguage.en,
+    kind: SourceKind.fun,
+    description: 'Webcomic of romance, sarcasm, math and language',
+    enabledByDefault: true,
+  ),
+  NewsSource(
+    id: 'thedailywtf',
+    name: 'The Daily WTF',
+    feedUrl: 'https://thedailywtf.com/rss',
+    homepage: 'https://thedailywtf.com',
+    language: SourceLanguage.en,
+    kind: SourceKind.fun,
+    description: 'IT horror stories, bad code and error messages',
+    enabledByDefault: true,
+  ),
+  NewsSource(
+    id: 'programmerhumor',
+    name: 'r/ProgrammerHumor',
+    feedUrl: 'https://www.reddit.com/r/ProgrammerHumor/top/.rss?t=day',
+    homepage: 'https://www.reddit.com/r/ProgrammerHumor/',
+    language: SourceLanguage.en,
+    kind: SourceKind.fun,
+    description: "The day's top programming memes on Reddit",
+  ),
 ];
 
 /// The curated sources before the app remembered which ones it had seen.