Add a darkened card when stack is empty

Closes #11

components/CardsGrouped.vue

@@ -19,6 +19,9 @@ const props = withDefaults(defineProps<Props>(), {
                     <slot></slot>
                 </CardPreview>
             </template>
+            <template v-if="props.stack.length == 0">
+                <img :class="`card_image empty`" :src="'/cards/background.png'" draggable="false">
+            </template>
         </div>
     </div>
 
@@ -38,4 +41,9 @@ const props = withDefaults(defineProps<Props>(), {
     flex-direction: row-reverse;
     justify-content: center;
 }
+
+.card_image.empty {
+    filter: brightness(0.5);
+    max-height: 250px;
+}
 </style>