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/form.css

@@ -124,18 +124,12 @@ textarea:focus,
 .ui.form .fields:not(.grouped):not(.inline) .field:not(:only-child) .ui.toggle.checkbox {
   margin-top: 2.21428571em;
 }
-.ui.form .fields:not(.grouped):not(.inline) .field:not(:only-child) .ui.slider.checkbox {
-  margin-top: 2.61428571em;
-}
 .ui.ui.form .field .fields .field:not(:only-child) .ui.checkbox {
   margin-top: 0.6em;
 }
 .ui.ui.form .field .fields .field:not(:only-child) .ui.toggle.checkbox {
   margin-top: 0.5em;
 }
-.ui.ui.form .field .fields .field:not(:only-child) .ui.slider.checkbox {
-  margin-top: 0.7em;
-}
 
 .ui.form .field > .selection.dropdown {
   min-width: 14em; /* matches the default min width */
@@ -289,15 +283,11 @@ textarea:focus,
   font-weight: var(--font-weight-medium);
   text-transform: none;
 }
-.ui.form .grouped.fields .field,
-.ui.form .grouped.inline.fields .field {
+.ui.form .grouped.fields .field {
   display: block;
   margin: 0.5em 0;
   padding: 0;
 }
-.ui.form .grouped.inline.fields .ui.checkbox {
-  margin-bottom: 0.4em;
-}
 
 .ui.form .fields {
   display: flex;