fix(helpdesk): hide zero-activity agents and add spacing in team dashboard

Filter out agents with 0 open tickets from Current tab and 0 resolved
from Performance tab to reduce noise. Add margin between period selector
and KPI card in the Performance panel.

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

modules/helpdesk/web/css/helpdesk.css

@@ -171,6 +171,10 @@
     color: var(--app-muted-color);
   }
 
+  .helpdesk-team-performance-panel > .helpdesk-segment-control {
+    margin-bottom: calc(var(--app-spacing) * 0.75);
+  }
+
   /* Muted zero values — de-emphasize non-information */
   .helpdesk-team-muted {
     color: var(--app-muted-color);