1. Use `is-loading` instead of `ui loader`
2. Introduce class name `image-diff-tabs`, instead of searching `gt-hidden`, which is fragile
3. Align the UI elements, see the screenshots.
There was some recent discussion about this in Discord `ui-design`
channel and the conclusion was that
https://github.com/go-gitea/gitea/issues/24305 should have fixed their
OS font installation to have semibold weights.
I have now tested this 601 weight on a Windows 10 machine on Firefox
myself, and I immediately noticed that bold was excessivly bold and
rendering as 700 because browsers are biased towards bolder fonts. So
revert this back to the previous value.
Ran most of the Less files through the Less compiler and Prettier and
then followed up with a round of manual fixes.
The Less compiler had unfortunately stripped all `//` style comments
that I had to restore (It did preserve `/* */` comments). Other fixes
include duplicate selector removal which were revealed after the
transpilation and which weren't caught by stylelint before but now are.
Fixes: https://github.com/go-gitea/gitea/issues/15565
