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/label.css

@@ -184,12 +184,6 @@ a.ui.ui.ui.basic.yellow.label:hover {
   border-color: var(--color-yellow-dark-1);
   color: var(--color-yellow-dark-1);
 }
-.ui.ui.ui.olive.label {
-  background: var(--color-olive);
-  border-color: var(--color-olive);
-  color: var(--color-white);
-}
-
 .ui.ui.ui.green.label {
   background: var(--color-green);
   border-color: var(--color-green);
@@ -221,16 +215,6 @@ a.ui.ui.ui.purple.label:hover {
   border-color: var(--color-purple-dark-1);
   color: var(--color-white);
 }
-.ui.ui.ui.basic.purple.label {
-  background: transparent;
-  border-color: var(--color-purple);
-  color: var(--color-purple);
-}
-a.ui.ui.ui.basic.purple.label:hover {
-  background: transparent;
-  border-color: var(--color-purple-dark-1);
-  color: var(--color-purple-dark-1);
-}
 
 .ui.ui.ui.grey.label {
   background: var(--color-label-bg);
@@ -242,16 +226,6 @@ a.ui.ui.ui.grey.label:hover {
   border-color: var(--color-label-hover-bg);
   color: var(--color-white);
 }
-.ui.ui.ui.basic.grey.label {
-  background: transparent;
-  border-color: var(--color-label-bg);
-  color: var(--color-label-text);
-}
-a.ui.ui.ui.basic.grey.label:hover {
-  background: transparent;
-  border-color: var(--color-label-hover-bg);
-  color: var(--color-label-hover-bg);
-}
 
 /* "horizontal label" is actually "fat label" which has enough padding spaces to be used standalone in headers */
 .ui.horizontal.label {