fix: fallback to standard mail ports when no exposure method configured

[ggueret]

Summary

When using an external ingress controller (e.g., Traefik with TCP routing), all built-in exposure methods are disabled (ingress.enabled, front.hostPort.enabled, front.externalService.enabled all set to false).

This results in an empty PORTS environment variable in the mailu-envvars ConfigMap, which can cause issues in the admin UI (see Mailu/Mailu#3944).

This PR adds a fallback to standard mail ports (25,465,587,143,993) when no ports are detected through the existing logic.

Changes

• Added fallback in mailu.enabledPorts helper when no exposure method is configured

Testing

helm template mailu charts/mailu \
  --set ingress.enabled=false \
  --set front.hostPort.enabled=false \
  --set front.externalService.enabled=false \
  | grep -A1 "PORTS:"

Before: PORTS: ""
After: PORTS: "25,465,587,143,993"

Fixes #548