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

static/js/api.js

@@ -88,3 +88,15 @@ class ApiClient {
 }
 
 window.api = new ApiClient();
+
+// Helper: waits until chartManager is available (defensive against load-order edge cases)
+window.waitForChartManager = async (timeoutMs = 3000) => {
+    if (window.chartManager) return window.chartManager;
+    const start = Date.now();
+    while (Date.now() - start < timeoutMs) {
+        await new Promise(r => setTimeout(r, 50));
+        if (window.chartManager) return window.chartManager;
+    }
+    console.warn('chartManager not available after', timeoutMs, 'ms');
+    return null;
+};