Show a failing feed on its artwork, in words, and grey (#93)

The mark was a 12px "!" in the sidebar's margin, told apart by --bad alone; a dark theme's --bad
is a pale pink, and at that size it vanished. It is now a solid disc on the artwork's corner, the
subtitle says what is wrong in place of the counts, and a feed failing for a day or more has its
artwork greyed out. Lightness and words carry it, so no theme's palette changes.

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

CHANGELOG.md

@@ -42,6 +42,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- A failing feed is easy to spot in any theme: a mark on its artwork, what is wrong in place of
+  its counts, and its artwork greyed out once it has failed for a day.
 - A feed whose own artwork is missing shows its website's icon instead of nothing.
 - The log has no terminal colour codes when it is not going to a terminal, as in `docker logs`.
 - A feed whose website names an icon that is missing shows the site's `/favicon.ico` instead of