Highlight viewed files differently in the PR filetree (#24956)

![image](https://github.com/go-gitea/gitea/assets/1135157/009d8026-15a1-4d18-8548-1c3642978f57)

fixes #24566

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/js/components/DiffFileTreeItem.vue

@@ -10,7 +10,7 @@
       />
       <a
         v-if="item.isFile"
-        class="file gt-ellipsis"
+        :class="['file gt-ellipsis', {'viewed': item.file.IsViewed}]"
         :href="item.isFile ? '#diff-' + item.file.NameHash : ''"
       >{{ item.name }}</a>
       <SvgIcon
@@ -148,4 +148,8 @@ a:hover {
   text-decoration: none;
   color: var(--color-text);
 }
+
+a.file.viewed {
+  color: var(--color-text-light-3);
+}
 </style>