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/main.py

@@ -1998,6 +1998,7 @@ async def get_app_config(request: Request):
     license_metadata = getattr(app.state, 'LICENSE_METADATA', None)
     user_count = await Users.get_num_users() if license_metadata else None
     config = await Config.get_many(
+        'oauth.enable',
         'oauth.auto_redirect',
         'ldap.enable',
         'ui.enable_signup',
@@ -2052,7 +2053,13 @@ async def get_app_config(request: Request):
         'version': VERSION,
         'default_locale': str(DEFAULT_LOCALE),
         'oauth': {
-            'providers': {name: config.get('name', name) for name, config in OAUTH_PROVIDERS.items()},
+            # Hide providers (and thus the login buttons / auto-redirect) when OAuth
+            # is disabled, without clearing the admin's provider configuration.
+            'providers': (
+                {name: provider.get('name', name) for name, provider in OAUTH_PROVIDERS.items()}
+                if config.get('oauth.enable', True)
+                else {}
+            ),
             'auto_redirect': config.get('oauth.auto_redirect'),
         },
         'features': {