feat: improve accessibility of scroll-to-top button (#179)

添加回到顶部按钮的aria-label值，提高无障碍评分

```release-note
None
```

templates/modules/layout.html

@@ -42,6 +42,7 @@
       th:if="${theme.config.global.show_scroll_button}"
       type="button"
       id="btn-scroll-to-top"
+      aria-label="回到顶部"
       class="fixed bottom-5 right-5 flex items-center justify-center rounded-full bg-white p-3 opacity-0 shadow transition-all duration-300 hover:opacity-70 hover:shadow-lg dark:bg-slate-600 lg:bottom-10 lg:right-10"
     >
       <span class="i-tabler-chevron-up text-2xl text-gray-600 dark:text-slate-200"></span>