Fix placeholder text of various empty tables and lists

dashboard/src/views/BackupSitesView.vue

@@ -220,7 +220,7 @@ onMounted(async () => {
 
       <div>
         <ProgressBar mode="indeterminate" v-if="busy" slim :show-label="false" />
-        <div v-if="!busy && sites.length === 0">{{ $t('backup.sites.emptyPlaceholder') }}</div>
+        <div v-if="!busy && sites.length === 0" class="empty-placeholder">{{ $t('backup.sites.emptyPlaceholder') }}</div>
         <div class="backup-site" v-for="site in sites" :key="site.id">
           <div style="display: flex; align-items: center;">
             <StateLED :busy="site.status.busy" :state="site.status.state"/>
@@ -249,6 +249,12 @@ onMounted(async () => {
 
 <style scoped>
 
+/* should match pankow-table-placeholder */
+.empty-placeholder {
+  text-align: center;
+  margin-top: 60px;
+}
+
 .backup-site {
   display: flex;
   border-radius: var(--pankow-border-radius);