enhance(actions): action view enhancements (#38594)

- avoid layout shift on actions log view page load
- retain header border when actions log is scrolled
- when copying step output, only copy with timestamp when timestamp
display is enabled (useful for example when step output is plain json
and copying with timestamp would make that json unparsable).

web_src/js/components/RepoActionView.vue

@@ -355,6 +355,7 @@ onBeforeUnmount(() => {
   flex-direction: column;
   gap: 4px;
   margin-top: 8px;
+  min-height: 50px; /* reserve the back link and title height so the body does not shift when the run data arrives */
 }
 
 .action-view-back {