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

static/js/pages/costs.js

@@ -91,6 +91,9 @@ class CostsPage {
 
     async loadCostsChart() {
         try {
+            const cm = window.chartManager || await window.waitForChartManager();
+            if (!cm) { this.showEmptyChart('costs-chart', 'Chart system unavailable'); return; }
+
             const data = await window.api.get('/usage/providers');
 
             if (!data || data.length === 0) {
@@ -107,7 +110,7 @@ class CostsPage {
                 }]
             };
             
-            window.chartManager.createBarChart('costs-chart', chartData);
+            cm.createBarChart('costs-chart', chartData);
             
         } catch (error) {
             console.error('Error loading costs chart:', error);