Fix all rounded borders, change affected tab menus to pills (#30707) (#30769)

Fix all rounded borders, change affected tab menus to pills (#30707) (#30769)

Backport #30707 by @silverwind

Fixes https://github.com/go-gitea/gitea/issues/30673, all 23 issues.
Notes:

- Tab bar menus had to change to pills because of unsolvable issue with
the border-radius as tab bar renders a overlapping border onto the box
below. And I think pills look better.
- Added padding to code editor empty preview message
- Hide monaco's built-in blue focus border, we don't need it and it
never showed before either.
- Label add menu is simplified, removing the nested segment.

<img width="1322" alt="Screenshot 2024-04-25 at 22 26 19"
src="https://github.com/go-gitea/gitea/assets/115237/7e394e0c-b7ad-417d-8e9f-12f1dea93ed1">
<img width="1326" alt="Screenshot 2024-04-25 at 22 28 00"
src="https://github.com/go-gitea/gitea/assets/115237/66c8499f-aa9f-4d95-8cca-ef13dfa82c65">
<img width="997" alt="Screenshot 2024-04-25 at 22 36 53"
src="https://github.com/go-gitea/gitea/assets/115237/07896102-c71d-4246-8173-c2bc2e1d3cae">
<img width="832" alt="Screenshot 2024-04-25 at 22 56 09"
src="https://github.com/go-gitea/gitea/assets/115237/d83afc96-08ca-4adc-baf4-3d02804be57c">
<img width="361" alt="Screenshot 2024-04-25 at 22 57 12"
src="https://github.com/go-gitea/gitea/assets/115237/c7371a68-00b5-47d8-84d0-ddc5268b2b2c">

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