feat: add a master OAuth / OIDC enable toggle in Authentication settings (#26988)

The OAuth / OIDC section in Admin Settings > Authentication had no
enable/disable switch, unlike the LDAP section above it. Add one that
persists via the existing Save flow and actually gates OAuth sign-in,
mirroring how the LDAP toggle works.

- config: new ENABLE_OAUTH persistent config ('oauth.enable'), defaulting
  to True so existing deployments with a provider configured keep working.
- oauth: expose ENABLE_OAUTH via the OAuth runtime config and reject the
  login and callback handlers with 404 when it is disabled.
- /api/config: report no OAuth providers when disabled so the login page
  hides the OAuth buttons (and cannot auto-redirect), without clearing the
  admin's provider configuration.
- auths: expose ENABLE_OAUTH through the admin OAuth config get/update
  endpoints (OAuthConfigForm + OAUTH_CONFIG_KEYS).
- Authentication.svelte: bind the OAuth / OIDC header Switch to the
  persisted oauthConfig.ENABLE_OAUTH and collapse the section when off,
  matching the LDAP header (size, weight, alignment).

backend/open_webui/routers/auths.py

@@ -1304,6 +1304,7 @@ class OAuthConfigForm(BaseModel):
     """All OAuth/OIDC settings exposed to the admin panel."""
 
     # General OAuth
+    ENABLE_OAUTH: bool | None = None
     ENABLE_OAUTH_SIGNUP: bool | None = None
     OAUTH_MERGE_ACCOUNTS_BY_EMAIL: bool | None = None
     OAUTH_AUTO_REDIRECT: bool | None = None
@@ -1359,6 +1360,7 @@ OAUTH_COMMA_LIST_FIELDS = {
 
 
 OAUTH_CONFIG_KEYS = {
+    'ENABLE_OAUTH': 'oauth.enable',
     'ENABLE_OAUTH_SIGNUP': 'oauth.enable_signup',
     'OAUTH_MERGE_ACCOUNTS_BY_EMAIL': 'oauth.merge_accounts_by_email',
     'OAUTH_AUTO_REDIRECT': 'oauth.auto_redirect',