Clean up CSS for API "connect" buttons

Instead of identifying each connect button by ID, we can just use the
.api_button class. The .menu_button class *would* override it due to CSS
cascade rules (specifically, declarations later in the stylesheet apply
over ones that appear earlier), but the `.menu_button.api_button`
selector has a higher *specificity* and hence works.

public/style.css

@@ -1444,9 +1444,7 @@ select option:not(:checked) {
     display: block;
 }
 
-#api_button:hover,
-#api_button_novel:hover,
-#api_button_textgenerationwebui:hover {
+.menu_button.api_button:hover {
     background-color: var(--active);
 }