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/org.css

@@ -31,21 +31,6 @@
   color: var(--color-text-light-3);
 }
 
-.organization.invite #invite-box {
-  margin: 50px auto auto;
-  width: 500px !important;
-}
-
-.organization.invite #invite-box #search-user-box input {
-  margin-left: 0;
-  width: 300px;
-}
-
-.organization.invite #invite-box .ui.button {
-  margin-left: 5px;
-  margin-top: -3px;
-}
-
 .organization.invite .ui.avatar {
   width: 100%;
   height: 100%;