Disallow deselecting timetable category

aleksis/apps/chronos/frontend/components/SelectTimetable.vue

@@ -76,7 +76,7 @@ export default {
       />
 
       <!-- Filter by timetable types -->
-      <v-btn-toggle v-model="selectedType" dense block class="d-flex">
+      <v-btn-toggle v-model="selectedType" dense block class="d-flex" mandatory>
         <v-btn
           v-for="type in types"
           :key="type.id"
@@ -126,9 +126,7 @@ export default {
         </v-list>
       </template>
       <template #loading>
-        <v-skeleton-loader
-          type="list-item-avatar@10"
-        />
+        <v-skeleton-loader type="list-item-avatar@10" />
       </template>
       <template #no-results>
         <div class="d-flex flex-column align-center justify-center">