Bug: `externalClusters` configuration ignored in standalone mode (regression from PR #760)

I'm running into an issue where my `externalClusters` configuration in `values.yaml` is being ignored when deploying a cluster in standalone mode. ``` mode: standalone # or not set (defaults to standalone) cluster: externalClusters: - name: source-db connectionParameters: host: "external-postgres.namespace.svc" port: "5432" user: "replication_user" dbname: "my_database" password: name: external-db-secret key: password ``` However, when I deploy the chart, the `externalClusters` section is completely missing from the generated Cluster resource. Looking at the template, I can see why this is happening. ### Root Cause In `charts/cluster/templates/_external_clusters.tpl`, the template logic is: ``` {{- if eq .Values.mode "standalone" }} {{- else }} externalClusters: ``` This means when `mode` is `standalone` (the default), the entire `externalClusters` section is skipped, even if I've explicitly configured custom external clusters in my values. I understand this was changed in PR #760 to fix Helm v4 compatibility issues with null values, which totally makes sense. However, this inadvertently removed the ability to configure custom external clusters in standalone mode, which is a legitimate use case for things like logical replication, foreign data wrappers, or monitoring external databases. ### Expected Behavior The chart should support custom `externalClusters` configuration in standalone mode. The `externalClusters` field should be rendered when explicitly configured, regardless of the mode.