Prevent layout shift in <overflow-menu> items (#29831)

There is a small layout shift in when active tab changes. Notice how the
actions SVG is unstable:


![](https://github.com/go-gitea/gitea/assets/115237/a6928e89-5d47-4a91-8f36-1fa22fddbce7)

This is because the active item with bold text is wider then the
inactive one. I have applied [this
trick](https://stackoverflow.com/a/32570813/808699) to prevent this
layout shift. It's only active inside `<overflow-menu>` because I wanted
to avoid changing HTML and doing it in regular JS would cause a flicker.
I don't expect us to introduce other similar menus without
`<overflow-menu>`, so that place is likely fine.


![after](https://github.com/go-gitea/gitea/assets/115237/d6089924-8de6-4ee0-8db4-15f16069a131)

I also changed the weight from 500 to 600, slightly reduced horizontal
padding, merged some tab-bar related CSS rules and a added a small
margin below repo-header so it does not look so crammed against the
buttons on top.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/css/repo/header.css

@@ -8,6 +8,7 @@
   flex-flow: row wrap;
   justify-content: space-between;
   gap: 0.5rem;
+  margin-bottom: 4px;
 }
 
 .repo-header .flex-item {