refactor(frontend): use useTemplateRef for DOM referencing

packages/frontend/src/components/MkVisitorDashboard.ActiveUsersChart.vue

@@ -13,7 +13,7 @@ SPDX-License-Identifier: AGPL-3.0-only
 </template>
 
 <script lang="ts" setup>
-import { onMounted, shallowRef, ref, nextTick } from 'vue';
+import { onMounted, useTemplateRef, ref, nextTick } from 'vue';
 import { Chart } from 'chart.js';
 import gradient from 'chartjs-plugin-gradient';
 import tinycolor from 'tinycolor2';
@@ -25,7 +25,7 @@ import { initChart } from '@/utility/init-chart.js';
 
 initChart();
 
-const chartEl = shallowRef<HTMLCanvasElement | null>(null);
+const chartEl = useTemplateRef('chartEl');
 const now = new Date();
 let chartInstance: Chart | null = null;
 const chartLimit = 30;