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>

README.md

@@ -180,6 +180,21 @@ page. Hacker News and Lobsters link to articles all over the web, so reader view
 depends on the linked site. Techmeme links to its own summary page, which only
 has the teaser; the original page links on to the full story.
 
+## Fun sources
+
+Tech humour for the Fun section of the Learn tab, which reaches back 3 days. Fun
+articles never show up in Today or the Briefing. ★ = enabled by default.
+
+| | Source | Feed | Focus |
+|---|---|---|---|
+| 🇬🇧 ★ | xkcd | `xkcd.com/atom.xml` | Webcomic of romance, sarcasm, math and language |
+| 🇬🇧 ★ | The Daily WTF | `thedailywtf.com/rss` | IT horror stories, bad code and error messages |
+| 🇬🇧 | r/ProgrammerHumor | `reddit.com/r/ProgrammerHumor/top/.rss?t=day` | The day's top programming memes on Reddit |
+
+Reader view shows comics and memes whole instead of as a cropped banner: xkcd
+with its title text underneath, The Daily WTF's Error'd with its screenshots.
+Reddit blocks reader view, so a meme shows the picture from the feed.
+
 ## Project layout
 
 ```