fix: add unified extensions button to top buttons in Zen UI manager; clean up vertical tabs CSS

src/browser/base/content/ZenUIManager.mjs

@@ -200,6 +200,7 @@ var gZenVerticalTabsManager = {
       for (const button of elements) {
         buttonsTarget.append(button);
       }
+      topButtons.appendChild(document.getElementById('unified-extensions-button'));
       topButtons.appendChild(document.getElementById('PanelUI-button'));
       if (this.isWindowsStyledButtons) {
         document.getElementById('zen-appcontent-navbar-container').append(

src/browser/base/content/zen-styles/zen-tabs/vertical-tabs.css

@@ -247,7 +247,6 @@
       margin-inline: var(--tab-block-margin);
     }
 
-    & #zen-sidebar-top-buttons,
     & #nav-bar {
       padding: var(--zen-element-separation);
       padding-right: 0;