fix(dashboard): add cache-busting and defensive chartManager guards to fix empty charts

static/js/pages/clients.js

@@ -75,6 +75,9 @@ class ClientsPage {
 
     async loadClientUsageChart() {
         try {
+            const cm = window.chartManager || await window.waitForChartManager();
+            if (!cm) { this.showEmptyChart('client-usage-chart', 'Chart system unavailable'); return; }
+
             const data = await window.api.get('/usage/clients');
             
             if (!data || data.length === 0) {
@@ -91,7 +94,7 @@ class ClientsPage {
                 }]
             };
             
-            window.chartManager.createHorizontalBarChart('client-usage-chart', chartData);
+            cm.createHorizontalBarChart('client-usage-chart', chartData);
             
         } catch (error) {
             console.error('Error loading client usage chart:', error);