[BUG] - `PORTS` environment variable empty in mailu-envvars ConfigMap

[ggueret]

Describe the bug

The mailu-envvars ConfigMap doesn't set the PORTS variable, leaving it empty. This causes issues with the admin UI when viewing domain details (see Mailu/Mailu#3944 for the related core fix).

To Reproduce

1. Deploy Mailu using the Helm chart with default values
2. Check the ConfigMap: kubectl get cm mailu-envvars -n mail -o jsonpath='{.data.PORTS}'
3. Observe the value is empty
4. Go to Admin UI > Domain > Details
5. See Internal Server Error (if running Mailu < 2024.06.45) or missing port information

Expected behavior

PORTS should be populated with the list of enabled mail ports (e.g., 25,465,587,143,993), derived from front.externalService.ports configuration.

Environment (please complete the following information):

• Helm Chart Version: 2.1.0
• Helm Version: v3.16.3
• Kubernetes Version: v1.31.4+k3s1
• Kubernetes Platform: K3s
• Mailu Version: 2024.06

Values.yaml Configuration

domain: domain.tld
hostnames:
  - mail.domain.tld

ingress:
  enabled: false

front:
  hostPort:
    enabled: false
  externalService:
    enabled: false

Additional information

Workaround:

kubectl patch cm mailu-envvars -n mail --type merge -p '{"data":{"PORTS":"25,465,587,143,993"}}'
kubectl rollout restart deploy/mailu-admin -n mail

Suggested fix: In the ConfigMap template, derive PORTS from enabled front.externalService.ports or add a dedicated Helm value.