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>

options/locale/locale_en-US.json

@@ -433,6 +433,7 @@
   "auth.authorize_application_created_by": "This application was created by %s.",
   "auth.authorize_application_description": "If you grant access, it will be able to access and write to all your account information, including private repos and organizations.",
   "auth.authorize_application_with_scopes": "With scopes: %s",
+  "auth.authorize_application_new_scopes": "New scopes: %s",
   "auth.authorize_title": "Authorize \"%s\" to access your account?",
   "auth.authorization_failed": "Authorization failed",
   "auth.authorization_failed_desc": "The authorization failed because we detected an invalid request. Please contact the maintainer of the app you tried to authorize.",