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/message.css

@@ -86,7 +86,6 @@ details.ui.message:not(:has(pre)) summary {
 .ui.error.message,
 .ui.attached.error.message,
 .ui.red.message,
-.ui.attached.red.message,
 .ui.negative.message,
 .ui.attached.negative.message {
   background: var(--color-error-bg);
@@ -95,9 +94,7 @@ details.ui.message:not(:has(pre)) summary {
 }
 
 .ui.warning.message,
-.ui.attached.warning.message,
-.ui.yellow.message,
-.ui.attached.yellow.message {
+.ui.attached.warning.message {
   background: var(--color-warning-bg);
   color: var(--color-warning-text);
   border-color: var(--color-warning-border);