Fix Unauthenticated: use local OIDCConfigEndpoint for embedded IdP

The management container was trying to fetch its own OIDC config via
the external URL (https://domain/oauth2/.well-known/...), which creates
a circular dependency: management -> DNS -> NPM -> Caddy -> management.
This fails because the management container can't reach itself through
the external network during startup.

Changed OIDCConfigEndpoint to http://127.0.0.1:80/oauth2/... (same as
LocalAddress) so the management server accesses its own embedded IdP
directly without going through DNS/NPM/Caddy.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

templates/management.json.j2

@@ -36,7 +36,7 @@
   "HttpConfig": {
     "AuthIssuer": "{{ external_url }}/oauth2",
     "AuthAudience": "netbird-dashboard",
-    "OIDCConfigEndpoint": "{{ external_url }}/oauth2/.well-known/openid-configuration"
+    "OIDCConfigEndpoint": "http://127.0.0.1:80/oauth2/.well-known/openid-configuration"
   },
   "EmbeddedIdP": {
     "Enabled": true,