fix(dashboard): guard customTooltip plugin against undefined tooltip and missing Y scale

chart.tooltip is undefined during initial draw for radial chart types
(doughnut/pie), and chart.scales.y doesn't exist on non-cartesian charts.
This crashed chart creation, causing 'Failed to load' messages on all pages.

static/js/charts.js

@@ -26,11 +26,14 @@ class ChartManager {
     }
 
     registerPlugins() {
-        // Register a plugin for tooltip background
+        // Register a plugin that draws a vertical dashed line at the active tooltip point.
+        // Only applies to charts with a cartesian Y scale (line, bar) — skip radial types.
         Chart.register({
             id: 'customTooltip',
             beforeDraw: (chart) => {
-                if (chart.tooltip._active && chart.tooltip._active.length) {
+                if (!chart.tooltip || !chart.tooltip._active || !chart.tooltip._active.length) return;
+                if (!chart.scales.y) return; // no cartesian Y axis (pie/doughnut)
+
                 const ctx = chart.ctx;
                 const activePoint = chart.tooltip._active[0];
                 const x = activePoint.element.x;
@@ -47,7 +50,6 @@ class ChartManager {
                 ctx.stroke();
                 ctx.restore();
             }
-            }
         });
     }