feat(helpdesk): add trend delta indicator per agent in team dashboard

Shows a small arrow next to the ticket count in each agent card header:
↑3 (orange) = received 3 more than resolved → backlog growing
↓2 (green) = resolved 2 more than received → backlog shrinking
Hidden when delta is 0 (stable).

Delta is computed from received_in_period (tickets created in the
selected period assigned to this agent) minus resolved_in_period.
No snapshot storage needed — derived from existing ticket data.

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

modules/helpdesk/web/css/helpdesk.css

@@ -220,6 +220,22 @@
     border-radius: 0;
   }
 
+  /* Team workload — trend delta indicator */
+  .helpdesk-team-trend-up,
+  .helpdesk-team-trend-down {
+    font-size: var(--text-xs, 0.75rem);
+    font-weight: 600;
+    font-variant-numeric: tabular-nums;
+  }
+
+  .helpdesk-team-trend-up {
+    color: var(--app-warning, #f59e0b);
+  }
+
+  .helpdesk-team-trend-down {
+    color: var(--app-success, #198754);
+  }
+
   /* Team workload — inline avatar for section titles */
   .helpdesk-team-avatar-inline {
     display: inline-flex;