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/header.css

@@ -24,10 +24,6 @@
   vertical-align: middle;
 }
 
-.ui.header > .ui.label.compact {
-  margin-top: inherit;
-}
-
 .ui.header .sub.header {
   display: block;
   font-weight: var(--font-weight-normal);