Fix fullscreen for action (#25200)

An error occurs when clicking on `show full screen` on action page.

<img width="1440" alt="Screen Shot 2023-06-12 at 13 06 52"
src="https://github.com/go-gitea/gitea/assets/17645053/1d4ded3c-fb77-4dd8-9201-24d0696f96eb">

class name has changed in #25134, so the selector is not working. 
Enhance the selectors to fix this.

Author: HesterG

web_src/js/components/RepoActionView.vue

@@ -415,7 +415,7 @@ const sfc = {
       const fullScreenEl = document.querySelector('.action-view-right');
       const outerEl = document.querySelector('.full.height');
       const actionBodyEl = document.querySelector('.action-view-body');
-      const headerEl = document.querySelector('.ui.main.menu');
+      const headerEl = document.querySelector('#navbar');
       const contentEl = document.querySelector('.page-content.repository');
       const footerEl = document.querySelector('.page-footer');
       toggleElem(headerEl, !this.isFullScreen);