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/templates/backend-deployment.yaml

@@ -79,7 +79,7 @@ spec:
                   name: {{ include "cloudhost-platform.secretName" . }}
                   key: jwt-refresh-secret
             - name: FRONTEND_URL
-              value: {{ include "cloudhost-platform.frontendPublicUrl" . | quote }}
+              value: {{ include "cloudhost-platform.corsOrigins" . | quote }}
             {{- range $key, $val := .Values.backend.env }}
             - name: {{ $key }}
               value: {{ $val | quote }}