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/js/helpdesk-team.js

@@ -110,6 +110,15 @@ if (container) {
     title.appendChild(document.createTextNode(name));
 
     title.appendChild(h('span', 'helpdesk-team-title-count', count));
+
+    // Trend delta: ↑3 (growing) or ↓2 (shrinking)
+    const delta = m.trend_delta ?? 0;
+    if (delta !== 0) {
+      const arrow = delta > 0 ? '↑' : '↓';
+      const cls = delta > 0 ? 'helpdesk-team-trend-up' : 'helpdesk-team-trend-down';
+      title.appendChild(h('span', cls, arrow + Math.abs(delta)));
+    }
+
     section.appendChild(title);
 
     // Ticket table