Improve "new-menu" (#24465)

I am not sure what "new-menu" means, but I think we need to fix these
problems:

1. it shouldn't have "stackable", which makes the items stacked when
width is small. the `new-menu` already has `overflow: auto`
2. `justify-content: center` doesn't work with `overflow: auto` (for
small width), so use `margin: auto`
*
https://bhch.github.io/posts/2021/04/centring-flex-items-and-allowing-overflow-scroll/
3. `runner-new-menu` is dead code (copying & pasting ?)

web_src/css/explore.css

@@ -1,5 +1,4 @@
 .explore .navbar {
-  justify-content: center;
   margin-bottom: 15px !important;
   background-color: var(--color-navbar) !important;
   border-width: 1px !important;