Show OAuth2 errors to end users (#25261)

Partially fix #23936


![image](https://github.com/go-gitea/gitea/assets/2114189/8aa7f3ad-a5f0-42ce-a478-289a03bd08a3)


![image](https://github.com/go-gitea/gitea/assets/2114189/bb901e7d-485a-47a5-b68d-9ebe7013a6b2)


![image](https://github.com/go-gitea/gitea/assets/2114189/9a1ce0f3-f011-4baf-8e2f-cc6304bc9703)

1 files changed, 6 insertions, 0 deletions

diff --git a/web_src/js/features/admin/common.js b/web_src/js/features/admin/common.js
index 84fd35e081..b6b192a296 100644
--- a/web_src/js/features/admin/common.js
+++ b/web_src/js/features/admin/common.js
@@ -171,6 +171,12 @@ export function initAdminCommon() {
     }
   }
 
+  if ($('.admin.authentication').length > 0) {
+    $('#auth_name').on('input', function () {
+      $('#oauth2-callback-url').text(`${window.location.origin}/user/oauth2/${encodeURIComponent($(this).val())}/callback`);
+    }).trigger('input');
+  }
+
   // Notice
   if ($('.admin.notice')) {
     const $detailModal = $('#detail-modal');