feat(panel): serve authenticated app on dedicated panel subdomain

Separate the marketing landing site from the authenticated app by host.
Next.js middleware reads PANEL_HOST/LANDING_HOST at runtime and redirects
authenticated routes (/dashboard, /login, /register) from the landing host
to the panel host, and the landing root on the panel host to /dashboard.
Disabled (single-origin) when PANEL_HOST is unset, so local dev is unchanged.

Helm: add ingress.panel.host with a third ingress rule + TLS host routing to
the frontend service, pass LANDING_HOST/PANEL_HOST to the frontend, and append
the panel origin to the backend CORS list (frontend URL stays first so
PLATFORM_DOMAIN resolution is unaffected).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/helm/cloudhost-platform/values.yaml

@@ -78,6 +78,10 @@ ingress:
   className: nginx
   frontend:
     host: platform.cloudhost.local
+  # Optional dedicated host for the authenticated panel. Empty = disabled
+  # (single-origin, dev). Production (abrban): panel.abrban.com
+  panel:
+    host: ""
   api:
     host: api.cloudhost.local
   singleHost: