Remove dead CSS (#37376)

Removes CSS rules that have zero usages across templates, Go source,
JS/TS/Vue, and `options/`. Each selector was cross-checked for runtime
additions (Fomantic JS, library classes) before removal.

A few rules with no current usages are kept as symmetric pairs of
heavily-used classes likely to be needed:
- `.ui.bottom.attached.header` / `.ui.bottom.attached.message` — pair
with the widely-used `top.attached` variants
- `.ui.warning.header` / `.ui.warning.segment` — warning-themed variants
of error-themed classes that are kept
- `.btn.small` — size variant alongside the kept `.btn.tiny`

---
This PR was written with the help of Claude Opus 4.7

---------

Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>

web_src/css/modules/card.css

@@ -122,14 +122,3 @@ a.ui.card:hover {
   color: var(--color-text);
   border-top-color: var(--color-secondary-light-1) !important;
 }
-
-.ui.three.cards {
-  margin-left: -1em;
-  margin-right: -1em;
-}
-
-.ui.three.cards > .card {
-  width: calc(33.33333333333333% - 2em);
-  margin-left: 1em;
-  margin-right: 1em;
-}