Manage display of book covers

front/src/AddBook.vue

@@ -8,7 +8,8 @@
 
   const book = ref({
     title: "",
-    author: ""
+    author: "",
+    coverId: null
   });
   const errors = ref(null)
   const titleError = computed(() => {
@@ -50,7 +51,7 @@
       </div>
       <p v-if="authorError" class="help is-danger">{{authorError}}</p>
     </div>
-    <CoverUpload name="cover"/>
+    <CoverUpload name="cover" @on-image-upload="(id) => book.coverId = id"/>
     <div class="field">
       <div class="control">
         <button class="button is-link">{{$t('addbook.submit')}}</button>