Fixes: https://github.com/go-gitea/gitea/issues/29981. Introduce
`.secondary-nav` as a universal way for styling and margin adjustments
inside `.page-content`.
If the first child of `.page-content` is `.secondary-nav`, we add margin
below it, otherwise we add padding to the first child. Notable changes:
- `--color-header-wrapper` is replaced with `--color-secondary-nav-bg`.
- `navbar` class is removed.
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Co-authored-by: Giteabot <teabot@gitea.io>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
1. Use general "mobile-only" and "not-mobile" CSS styles, remove some`@media (max-width: 767.98px)` tricks
2. Use `CountFmt` for repo list, just like the repo header (and it matches GitHub, to avoid big numbers bloat the page)
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.
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Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>