fix(oauth2): allow users to approve scope changes (#38942) (#39521)

Backport #38942 by @Harsh-128

Lets users approve an OAuth2 scope change on an existing grant instead
of failing with `a grant exists with different scope`. Fixes
https://github.com/go-gitea/gitea/issues/38940.

- Approving a different scope updates the existing grant. Issued tokens
follow immediately, since their scope is read from the grant.
- Confidential and trusted apps show the consent page when the scope set
changes, instead of silently reusing the old grant.
- An omitted `scope` reuses the existing grant's scope, like GitHub.
- The consent page lists newly added scopes.

<img width="500" alt="consent page with new scopes"
src="https://github.com/user-attachments/assets/40282353-32fe-4d87-9929-08aabbab32f1"
/>

Co-authored-by: Harsh Sharma <harshee2000@gmail.com>
Co-authored-by: bircni <bircni@icloud.com>
Co-authored-by: silverwind <me@silverwind.io>

templates/user/auth/grant.tmpl

@@ -12,6 +12,7 @@
 				{{end}}
 				{{ctx.Locale.Tr "auth.authorize_application_created_by" .ApplicationCreatorLinkHTML}}<br>
 				{{ctx.Locale.Tr "auth.authorize_application_with_scopes" (HTMLFormat "<b>%s</b>" .Scope)}}
+				{{if .AddedScopes}}<br>{{ctx.Locale.Tr "auth.authorize_application_new_scopes" (HTMLFormat "<b>%s</b>" (StringUtils.Join .AddedScopes " "))}}{{end}}
 			</p>
 		</div>
 		<div class="ui attached segment">