Keep the feed list's icons when it redraws, rather than load each again (#134)

Checking every feed made all the feeds' icons in the feed list flash while it ran. A check sends
each feed's new row as it reads the feed, and renderFeeds, run once a frame while they come in,
emptied the list and built every row anew, every <img> with it: each icon went blank and was
loaded and drawn again, many times a second through a scan of 150 feeds.

The rows are still rebuilt, but the images already drawn go into the new ones in place of the
fresh copies, matched by their markup, so only an icon that has changed is made anew. A browser
test checks the rows are new and the images in them the same elements, and fails without this.

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

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- Checking every feed no longer makes every icon in the feed list flash while it runs: the list keeps the icons it has already drawn.
+
 ## [0.10.0] - 2026-10-05
 
 ### Added