This will move the settings button back to the right, like known from
older versions.
For this, the overflow-menu was changed when a setting button is
available. If no settings button is available, the behavior will not
change.
Fixes #3301
There is a small layout shift in when active tab changes. Notice how the
actions SVG is unstable:

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.

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>
---
Conflict resolution: Moved an `:focus` selector to the new CSS rule.
Ref: https://codeberg.org/forgejo/forgejo/issues/2776
(cherry picked from commit 99d7ef50917e8d61798715e1b0b3dc1a99709f27)