Tweak actions view sticky (#29781)

Add some space when the left side items are sticky due to scrolling the
right side.

<img width="419" alt="image"
src="https://github.com/go-gitea/gitea/assets/115237/292e1b03-a071-4744-bb79-e50d109056c8">

web_src/js/components/RepoActionView.vue

@@ -524,7 +524,7 @@ export function initRepositoryActionView() {
   width: 30%;
   max-width: 400px;
   position: sticky;
-  top: 0;
+  top: 12px;
   max-height: 100vh;
   overflow-y: auto;
 }