Show the mismatched ROOT_URL warning on the sign-in page if OAuth2 is enabled (#25947) (#25972)

Backport #25947 by @wolfogre

Since OAuth2 will callback the root URL, if the user starts signing in
from a wrong host, Gitea will return 500 because it cannot find the
session.

<details>
<summary>How to reproduce</summary>

<img width="901" alt="image"
src="2c2e255c-e13e-4a11-9be7-b226bee54920">

<img width="1014" alt="image"
src="b31cfcf6-a320-483d-9ce5-ba8562f065e1">

</details>

So show the mismatched ROOT_URL warning on the sign-in page if OAuth2 is
enabled.

<img width="1015" alt="image"
src="99e80b17-c790-49a3-bbf2-2bd9396a7daa">

Co-authored-by: Jason Song <i@wolfogre.com>
(cherry picked from commit 037a3f0d8c)

web_src/js/features/common-global.js

@@ -373,5 +373,5 @@ export function checkAppUrl() {
     return;
   }
   showGlobalErrorMessage(`Your ROOT_URL in app.ini is "${appUrl}", it's unlikely matching the site you are visiting.
-Mismatched ROOT_URL config causes wrong URL links for web UI/mail content/webhook notification.`);
+Mismatched ROOT_URL config causes wrong URL links for web UI/mail content/webhook notification/OAuth2 sign-in.`);
 }