According to [Debian
docs](https://wiki.debian.org/DebianRepository/UseThirdParty):
> The certificate MUST NOT be placed in /etc/apt/trusted.gpg.d or loaded
by apt-key add.
> ...
> If future updates to the certificate will be managed by an apt/dpkg
package as recommended below, then it SHOULD be downloaded into
/usr/share/keyrings using the same filename that will be provided by the
package. If it will be managed locally , it SHOULD be downloaded into
/etc/apt/keyrings instead.
> ...
> A sources.list entry SHOULD have the signed-by option set.
This PR is an extended implementation of #25189 and builds upon the
proposal by @hickford in #25653, utilizing some ideas proposed
internally by @wxiaoguang.
Mainly, this PR consists of a mechanism to pre-register OAuth2
applications on startup, which can be enabled or disabled by modifying
the `[oauth2].DEFAULT_APPLICATIONS` parameter in app.ini. The OAuth2
applications registered this way are being marked as "locked" and
neither be deleted nor edited over UI to prevent confusing/unexpected
behavior. Instead, they're being removed if no longer enabled in config.

The implemented mechanism can also be used to pre-register other OAuth2
applications in the future, if wanted.
Co-authored-by: hickford <[email protected]>
Co-authored-by: wxiaoguang <[email protected]>
---------
Co-authored-by: M Hickford <[email protected]>
Co-authored-by: wxiaoguang <[email protected]>
Our `devtest` page contains some good UI examples which should be taken
as example when working on Gitea UI. Mention it in our frontend
contributing guidelines to for new contributors to know about it.
The PKCE flow according to [RFC
7636](https://datatracker.ietf.org/doc/html/rfc7636) allows for secure
authorization without the requirement to provide a client secret for the
OAuth app.
It is implemented in Gitea since #5378 (v1.8.0), however without being
able to omit client secret.
Since #21316 Gitea supports setting client type at OAuth app
registration.
As public clients are already forced to use PKCE since #21316, in this
PR the client secret check is being skipped if a public client is
detected. As Gitea seems to implement PKCE authorization correctly
according to the spec, this would allow for PKCE flow without providing
a client secret.
Also add some docs for it, please check language as I'm not a native
English speaker.
Closes#17107Closes#25047