v3.28.8: the batch panel's scroll preservation

The attempts box's content is replaced on every render (the batch panel is
static markup; the render fills the dynamic bits) and replacing a
scrollable element's content resets its scroll.  That was harmless while
the render ran once per attempt, but v3.28.7 made it run on every 2 s
poll, so scrolling back through the attempts during a run snapped to the
top each poll.

The render now captures the box's scrollTop before refilling it and
restores it afterwards.

Test: the stub box resets its scroll on an innerHTML write (like a
browser) and the render must keep the reader's position - verified to
fail without the restore.  920 frontend + 928 Python; sw cache
simple-v446.

frontend/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'simple-v445';
+const CACHE = 'simple-v446';
 const URLS = [
   'index.html',
   'help.html',