Resolve "Add export functionality to course book"

Closes #258 (closed)

aleksis/apps/alsijil/frontend/components/coursebook/statistics/StatisticsAbsencesCard.vue

@@ -4,6 +4,7 @@
     <v-card-text class="d-flex flex-column" style="gap: 0.5em">
       <absence-reason-chip
         v-for="absenceReason in absenceReasons"
+        :key="absenceReason.id"
         :absenceReason="absenceReason.absenceReason"
         :count="absenceReason.count"
       />