Remove dead CSS rules (#37173) (#37177)

Backport #37173 by @silverwind

Remove CSS rules whose HTML classes/IDs are no longer referenced in any
template, Go source, or JavaScript/TypeScript file:

- `.archived-icon`: removed from templates in c85bb62635
- `.bottom-line`: removed from blame rendering in 9c6aeb47f7
- `.commit-status-link`: removed from templates in f3c4baa84b
- `.instruct-toggle`: removed from templates in 75e85c25c1
- `.runner-new-text`, `#runner-new`: never referenced outside CSS
- `.ap-terminal`: stale, asciinema-player uses `.ap-term`, still not
needed
- `.scrolling.dimmable.dimmed`: dimmer stand-in never adds this class
- `.markup span.align-center/align-right/float-left/float-right`: never
produced by any renderer, sanitizer strips class attributes
- `.markup ul.no-list`, `.markup ol.no-list`: same as above

---
This PR was written with the help of Claude Opus 4.6

Signed-off-by: silverwind <me@silverwind.io>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Claude (Opus 4.6) <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/css/modules/modal.css

@@ -159,19 +159,11 @@
   display: block;
 }
 
-.scrolling.dimmable.dimmed {
-  overflow: hidden;
-}
-
 .scrolling.dimmable > .dimmer {
   justify-content: flex-start;
   position: fixed;
 }
 
-.scrolling.dimmable.dimmed > .dimmer {
-  overflow: auto;
-}
-
 .modals.dimmer .ui.scrolling.modal {
   margin: 2rem auto;
 }