web/admin: add fallback font for doughnut charts

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

web/src/elements/charts/Chart.ts

@@ -99,7 +99,7 @@ export abstract class AKChart<T> extends LitElement {
                     const height = chart.height || 0;
 
                     const fontSize = (height / 114).toFixed(2);
-                    chart.ctx.font = `${fontSize}em Overpass`;
+                    chart.ctx.font = `${fontSize}em Overpass, Arial, sans-serif`;
                     chart.ctx.textBaseline = "middle";
                     chart.ctx.fillStyle = this.fontColour;