fix(helpdesk): replace generic loading with card-shaped skeleton placeholders

Skeleton loading now mirrors the actual agent card layout: bordered card
with header (avatar circle + name bar + badge) and three table-like rows.
Removes aria-busy toggle from JS. Three skeleton cards shown during load.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

modules/helpdesk/web/js/helpdesk-team.js

@@ -29,7 +29,6 @@ if (container) {
     if (elError) elError.hidden = state !== 'error';
     if (elEmpty) elEmpty.hidden = state !== 'empty';
     if (elContent) elContent.hidden = state !== 'success';
-    if (elLoading) elLoading.setAttribute('aria-busy', state === 'loading' ? 'true' : 'false');
   }
 
   function h(tag, cls, text) {